This is simple Pythagoras. You are given two sides, so you do:
\(c^{2} - a^{2} = b^{2}\)
(13 is c, and 7 is a)
So:
13² - 7² = c²
c² = 120
√120 = 10.95
c = 10.95
However, they haven't given you this option, so I think you just leave it in terms of c², which is D. 120cm.
