Geometry: Finding a surface that is orthogonal to a given family of surfaces Problem
Consider a one-parameter family of surfaces ${S_c}$ in $\mathbb{R^3}$ (with $c$ being a real constant) described by
$$S_c = \{(x, y, z)|f(x, y, z) = c\}, c ∈ \mathbb{R}$$
(a) Assume that we seek the surface $T$ described as the graph of a function of $x$ and
$y$ (i.e., $z = \phi(x, y)$). Show that the graph
$$\{(x, y, \phi(x, y)) | x, y ∈ \mathbb{R}\}$$
is orthogonal to each $S_c$ if and only if the function  satisfies the PDE
$$f_x(x, y, \phi(x, y))\phi_x + f_y(x, y, \phi(x, y))\phi_y = f_z(x, y, \phi(x, y))$$
(b) Use the method of characteristics to find the surface that intersects each of the
surfaces
$$z(x + y) = c(3z + 1)$$
orthogonally, and which contains the circle $x^2 + y^2 = 1, z = 1$. Here, $c ∈ \mathbb{R}$ just like above. 
My Attempt we know that two planes are orthogonal to each other if the dot product of their normal vectors is zero. I think this is how part (a) should be approached, also because the PDE contains the terms $f_x$,$f_y$, and $f_z$ such that $<f_x,f_y, f_z>$ is one of the normal vectors and $<\phi_x,\phi_y, 1>$ is the other, but I am not sure how to proceed. Help is extremely appreciated. Thanks!!

I can help just with question $(a)$ since I don't know about the "characteristics" method.
First of all notice that, in general, the normal vector at a point $P$ of a surface defined by the equation $g(x,y,z)=0$ is given by the gradient of $g$ evaluated at $P$:
$$ \nabla g(P) = [\;\partial_x g(P),\,\partial_y g(P),\, \partial_z g(P) \;] $$
(Let me know if the reason behind this statement is not clear for you, and I will elaborate).
In our case the surfaces have equations:
$$ f(x,y,z)-c=0 \quad\text{ and }\quad \phi(x,y)-z=0 $$
which lead to the gradients
$$ 
V = [\; \partial_x\, f,\,\partial_y\, f ,\,\partial_z\, f \;] 
\quad\text{ and }\quad 
W = [\; \partial_x \phi,\,\partial_y \phi ,\, -1 \;]
$$
Hence the $(a)$ follows, since $V(P)\cdot W(P) = 0 \iff$ your PDE holds, for every point $P$ of $T$.
