#### Answer

x=-3 or 7

#### Work Step by Step

Rewrite the equation so it is in standard form.
x(x-4)=21
x$^2$-4x=21
x$^2$-4x-21=0
Factor the trinomial.
(x+3)(x-7)=0
Set each variable factor as equal to zero and then isolate the variable to find the solutions.
x+3=0 or x-7=0
x=-3 or x=7
Enter the equation into your graphing utility to observe that the graph crosses the x-axis at-3 and 7.