This is a classic question testing your understanding of Pythagoras' Theorem. This is an equation which shows us the relationship between the 3 sides of a right angled triangle : a2 + b2 = c2. a and b are the shorter sides, while c is the longest (the hypotenuse).For this question, we have a and c, but not b. If we subtract a from the left side of the equation and add it to the right side, we have b2 = c2 - a2. Now we can fill in our values: b2 = 52 - 32, or b2 = 16. Now we take the square root of both sides, and find that b = 4cm.