Solve this equation by completing squares and then check the solution..
x = 21/x-4
x = 21/(x-4)
just by inspection, since the factors of 21 are 3 and 7, with one 4 less than the other, that's what I'd expect for roots of the equation.
x(x-4) = 21
x^2-4x = 21
x^2-4x+4 = 21+4
(x-2)^2 = 25
x-2 = ±5
x = 2±5
That is, 7 or -3
We can't use a negative value, so x=7