Answer
5
All you need to know to solve this problem is the pythogorean theorem. If you draw an imaginary line to make a triangle and a rectangle, you already know the height is 4. To find the base, you know that the bottom length is 9 and the top is 6 so 9-6 =3, which is the length of only the left side of the imaginary line.
So 4^2 + 3^2 = c^2
25 = c^ 2
5 = c
Hope that helps!