Solve using the quadratic formula

Question

X^2-4x=21

Answer 1

Learn the quadratic formula:

x = [-b +/-sqrt(b^2 -4ac)]/2a

In your case a = 1, b = -4 and c = -21

x = [4 +/-sqrt(100)]/2

You finish it. You surely know what the square root of 100 is.
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