Question 8163
You can use the Pythagorean theorem to find the missing length.
{{{c^2 = a^2 + b^2}}}  C is the hypotenuse (20), a is the height (15) and b is the missing length.

{{{20^2 = 15^2 + b^2}}}

{{{b^2 = 400 - 225}}}

{{{b = sqrt(175)}}} = {{{sqrt(25*7)}}}

{{{b = 5sqrt(7)}}} The exact answer.

{{{b = 5(2.64575)}}} = 13.228756...  The approximate answer.