Answer
$-3,7$
Work Step by Step
Given: $x(x-4)=21$
or, $x^2-4x-21=0$
Need to use factorization to solve the quadratic equation.
This can be factorized as follows:
$x^2-7x+3x-21=0$
or, $(x+3)(x-7)=0$
Now, $x+3=0$ and $x-7=0$
This implies $x=-3,7$
