The side with all the 9s and x is 12 just like its opposite side because this is a parallelogram. We can write the equation to add up the three segments; the ones on the left and right are 9-x:
9-x + x + 9-x = 12
18 - x = 12
x = 6
A^2 + b^2 = c^2
3^2 + 8^2 = c^2
9 + 64 = c^2
73 =c^2
take the square root of each side
c= 8.544