Solve for x. Solving square root equations often results in extraneous solutions, or answers that don't work in the original equation, so be sure to check your work.
a. 3 + √x - 4 = 20
b. √x + 7 = - 3
c. 4 - (x - 2)2 = - 21
d. 5 - √-(x + 4) = 2