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.