Question 1019394
The Distance Formula, using points (h,k) and (p,r):
{{{sqrt((h-p)^2+(k-r)^2)}}}


The diagonals in your graph are the point pairs (-b,c) and (a,0);
and (b,c) and (-a,0).


Substitute into the formula to make the expression for each diagonal.  These are
{{{sqrt((-b-a)^2+(c-0)^2)}}}  and  {{{sqrt((b-(-a))^2+(c-0)^2)}}}.
Simplify these.  (BASIC ALGEBRA SKILLS)