Answer:
15
Explanation:
Let's name every side.
AB = 11
AD = 12
CD = 20
We should find out what is the value of BC.
We can draw height from point B - BK.
Let's look at triangle BCK. It is right triangle. To find CK, we may do: CK = 20 - 11 = 9. And we know that BK = AD = 12. So, we can find hypotenuse by Pythagorean theorem.
x² = 144 + 81 = 225 => x = 15