You use the Pythagorean theorem .
Formula: a^2 + b^2 = c^2
Solve for x:
Take out exponent.
12^2 + x^2 = 20^2
Subtract 144 on both sides to get the variable alone.
144 + x = 400
-144 -144
You get the solution as 256. But you know it's bigger than the numbers given. So find the square root of 256.
x = 256
Which leaves you with:
x = 16
Check:
12^2 + 16^2 = 20^2
144 + 256 = 400
400 = 400
Key:
a = First shortest side (shorter side with the number smaller than side b and c)
b = Second shortest side (the number that is between side a's and c's number)
c = Third shortest side (longer side with the number larger than side a and b)