Maths problem: how to do, thanks.?

mx(x-2)+x²  ≡ nx(x+1) - 4x

I can find m & n by comparing coeff. of x² & x terms on both sides, as follows :-

Comparing coefficients of x² terms on both sides :
m+1 = n . . . . . . . . . . . ① 
Comparing coefficients of x terms on both sides :
-2m = n-4 . . . . . . . . . . ② 

①-②: 3m+1=4 => m=1
Put m=1 into ①: 1+1= n => n=2