We need to use the Pythagorean Theorem.
It states that a^2 + b^2= c^2, where a and b are the legs of the right angle triangle and c is the hypotenuse.
13 is the hypotenuse, 7 and b are the legs.
[tex]7^2 + b^2 = 13^2\\\\49 + b^2 = 169\\\\b^2 = 169 - 49\\\\b^2 = 120\\\\b=\sqrt{120} = \sqrt{4 \cdot 30} = \boxed{\bf{2\sqrt{30}}}[/tex]
The value of b is 2√(30).
