To solve this, use pythagorean theroem:
a^2 + b^2 = c^2
Since the diagonal is across from the right angle, it is represented as c.
10^2 + 15^2 = c^2
100 + 225 = c^2
325 = c^2
sqrt 325 = c
Converting to simplest radical form:
5 * sqrt 13 = c
Hope this helps!