Question 1008984
There are several different ratios that make right triangles. One set is 3:4:5.

This is a problem that fits that.

9:12:15  is 3:4:5   so the answer is 15cm.

You can also solve using Pythagorean theorem
{{{a^2 + b^2 = c^2}}}
{{{9^2 + 12^2 = c^2}}}
{{{81 + 144 = c^2}}}
{{{225 = c^2}}}
{{{15 = c}}}