Take half of the coefficient of the bx term and square on both sides:
x^2 - 10x + (-10/2)^2 = 46 + (-10/2)^2
x^2 - 10x + (-5)^2 = 46 + (-5)^2
x^2 - 10x + 25 = 46 + 25
x^2 - 10x + 25 = 71
Now factor x^2 - 10x + 25 which will give us (x - 5)^2
(x - 5)^2 = 71 is the answer.
