Answer:
We can find the solution using chain rule,
For instance, if u(x,y) = f(ax+by), then
which represents derivative of x with respect to x
and then derivative of
with respect to y is
,
Now, the derivative of
with respect to x, which is the second derivative, which is

and the derivative
and
are
,

Finally, the solution of PDE is


= 0,
As the PDE is equal to 0, it means the function u(x,y) = f(ax+by) is the solution of the given PDE.