The work out the length of the missing side you need to use Pythagoras' theorem which states that the sum of the squares of the two sides, in this case x and 60, is equal to the sum of the square of the hypotenuse. This can be formulated as

. In this case x can replace a or b (I've replaced a).
To solve for x rearrange the equation by subtracting

from both sides:

Substitute the numbers in:
= 3721-3600
= 121
Take the square root of 121 to find x:
x =

= 11cm