Question 38398
The general idea is to isolate x on one side of the equation and put everything else on the other side...to do that, you normally try to UNDO what is being done to x...so in this problem
x^2 + 7 = 21
you want to undo the +7 by subtracting 7 from both sides first, giving us
x^2 = 14
now you want to take the square root of both sides.  This undoes the squaring of x, and we get
x = ± sqrt(14)
Two answers, a plus and a minus square root of 14.