x - 12√x + 36 = 0
(√x)² - 2*6√x + 6² = 0
We have a perfect square here. (a-b)² =a² - 2ab + b²,
where a= √x and b = 6,
so for our equation we can write
(√x)² - 2*6√x + 6² = (√x-6)²
(√x-6)² = 0
√x-6 = 0
√x = 6
(√x)² = 6²
x = 6²=36
Check:
x - 12√x + 36 = 0
36 - 12√36+ 36 = 0
36 -12*6 +36=0
72 - 72 = 0
0=0 True
Answer is B. 6²
