Q:

Exercise 1.105 of Megginson's "An Introduction to Banach Space Theory"

Let $X$ be the vector space $\mathbb{R}^2$ and let $Y$ be the subspace of $X$ consisting of
all $(\alpha, \beta)$ such that $\beta = 0.$ Define $ f_0 = Y \rightarrow \mathbb{R}  $ by the formula $f_0(\alpha,0)=\alpha. $
Notice that $f_0$ is a bounded linear functional on $Y$ with respect to any
norm given to $X$.
(a) Suppose that $X$ is given the Euclidean norm. Show that $f_0$ has a
unique Hahn-Banach extension to $X$.
(b) Suppose that $X$ is given the norm that makes it into $l_1^2$. Show that
$f_0$ has an infinite number of different Hahn-Banach extensions to $X$.

Attempted proof
(a) If $f_0$ is to have any linear extension $f: \mathbb{R}^2 \rightarrow \mathbb{R} $, this extension should look like $f(x,y)=ax + by $ for all $(x,y) \in \mathbb{R}^2$. Thus $\lVert f \rVert = \sqrt{a^2 + b^2} $ (though I'm not quite sure about this. A quick argument for it will be appreciated). By the Hahn-Banch Theorem, since the operator norm of $f_0$ should equal the operator norm of $f$, we must have 
  \begin{equation}
1 = \sqrt{a^2 + b^2} 
\end{equation}
  Again, by the Hahn-Banch Theorem, since the restriction of $f$ to $Y$ should equal $f_0$ we must have for arbitray $x$:
  $$f\lvert_Y = f_0 \Rightarrow ax+by\rvert_{y=0} = x \Rightarrow ax = x\Rightarrow a=1 $$
  It follows that $b=0$, and since this solution is unique, we've found the uniqe extention to $X$, which is $f(x,y) = x$ for all $(x,y) \in \mathbb{R}^2$.

For part (b) I have absolutely no idea. Any help will be appreciated.

A:

For $(a)$, 
\begin{align}
\|f\|&=\sup\{|ax+by|:\ x^2+y^2=1\}.
\end{align}
By Cauchy-Schwarz, $$|ax+by|\leq\sqrt{a^2+b^2}\,\sqrt{x^2+y^2},$$ and the equality is achieved. So $\|f\|=\sqrt{a^2+b^2}$. 
For part $(b)$, we still have $f(x,y)=ax+by$. But now
$$
\|f\|=\sup\{|ax+by|:\ |x|+|y|=1\}=\max\{|a|,|b|\}.
$$
So $f_b(x,y)=x+by$ is a Hahn-Banach extension for all $b$ with $|b|\leq1$. 

