Prob. 5 (a) in Supplementary Exercises in Munkres' TOPOLOGY, 2nd ed: How to show that this map is a homeomorphism? Let $G$ be a topplogical group, and let $H$ be a subgroup of $G$. Let $G / H $ denote the collection of all left cosets of $H$ in $G$, and let $a$ be a fixed element of $G$. Let the map $f \colon G / H \to G / H$ be defined as follows: 
$$f(xH) \colon= axH \ \ \ \mbox{ for all } \ xH \in G/H.$$
Then how to show that the map $f$ is a homeomorphism? 
Here $G/H$ has the quotient topology. 
My effort: 
For any two elements $x, y \in G$, we have $xH = yH$ if and only if $x^{-1}y \in H$. 
So if $xH = yH$, then $x^{-1}y \in H$; so
$$(ax)^{-1}(ay) = (  x^{-1}a^{-1})(ay) = x^{-1}y \in H,$$
showing that $axH = ayH$. So $f$ is well-defined (i.e. single-valued). 
Now if $f(x) = f(y)$, then $axH = ayH$ and so $(ax)^{-1}(ay) \in H$. But 
$$(ax)^{-1}(ay) = (  x^{-1}a^{-1})(ay) = x^{-1}y. $$
So $x^{-1}y \in H$, which implies that $xH = yH$. Thus $f$ is injective. 
Now if $yH \in G/H$, then $y \in G$ and so $a^{-1}y \in G$ also; therefore $a^{-1}y H \in G/H$. Moreover, 
$$f(a^{-1}yH) = aa^{-1}yH = yH,$$
showing that $f$ is surjective. 
Hence $f$ is bijective. 
How to proceed from here? How to show that $f$ is continuous? And, how to show that $f$ is open?
I know that the map $f_a \colon G \to G$ defined by 
$$f_a(x) \colon= ax \ \ \ \mbox{ for all } \ x \in G$$
is a homeomorphism. 
And, the map $p \colon G \to G/H$ defined by 
$$p(x) \colon= xH \ \ \ \mbox{ for all } \ x \in G$$
is surjective. 
Moreover, the action of the map  $f$ on the elements of $G/H$ is in some way similar to the action of the map $f_a$ on the elements of $G$. 
How to proceed from here? 

Let us first show that $f$ is continuous: given $U \subset G/H$ open, $f^{-1}(U)$ is open iff $p^{-1}(f^{-1}(U))$ is open, where $p \colon G \to G/H$ is the quotient map. However, 
$$
p^{-1}(f^{-1}(U)) = (f \circ p)^{-1}(U) = (p \circ \tilde{f})^{-1}(U)
$$
is open, where $\tilde{f} \colon G \to G$ is left-multiplication by $a$. This is because $p \circ \tilde{f} \colon G \to G \to G/H$ is a composition of continuous maps, hence continuous. Moreover, the same argument shows that the map $g \colon G/H \to G/H$ given by $xH \mapsto a^{-1} xH$ is continuous, and it is clearly the inverse of $f$, so $f$ is a homeomorphism.
