3/4
The easiest way to solve this is to test the numbers one by one. 2/3 is not the answer because 2/3 times 3/2 is 1, not 9/8. It is 3/4 because 3/2 times 3/4 is 9/8, 9/8 times 3/4 is 27/32, and so forth.
X = SQRT( 4 * (12+4))
x - sqrt( 4 * 16)
x = sqrt(20)
x = 2 sqrt(5)