Question 134696
Use the Pythagorean theorem

For right triangles, a^2 + b^2 = c^2
Your sides are 8 and 15. The diagonal would be the hypotenuse of that triangle.

{{{8^2 + 15^2 = c^2}}}
{{{64 + 225 = c^2}}}
{{{289 = c^2}}}
{{{sqrt(289) = c}}}
{{{17 = c}}}