Question 186849
Let width be w. Then Length is (w+3)
{{{a^2 + b^2 = c^2}}} pythagorean theorem
{{{w^2 + (w+3)^2 = 15^2}}}
{{{w^2 + w^2 + 6w + 9 = 225}}}
{{{2w^2 + 6w - 216 = 0}}}
{{{2(w-9)(w+12) = 0}}}
w = 9 or w = -12. Since a width cannot be negative, then w = 9. And length = 12

Notice the triangle formed by width:length:diagonal is in the ration 3:4:5  -- a common ratio for right triangles and one you would do well to 'recognize on sight'.