Question 925562
w for width, L for length, d for the size of the diagonal.


A rectangle with dimensions L and w and diagonal d forms two right triangles allowing for the Pythagorean Theorem arithmetic statement {{{w^2+L^2=d^2}}}.


According to your example description, L=w+3 and d=15.
{{{highlight_green(w^2+(w+3)^2=15^2)}}};
take this the rest of the way.


(You should find along the way, {{{highlight_green(w^2+3w-108=0)}}} .)