An algebra problem by Yujie Cai

Since the calculation of our expression $(2mx^2-x^2+3x+1$ ) - ( $5x^2-5y^2+3x$ ) should be independent of $x$ , $m=3$ so the $x^2$ can be canceled. Plugging $m=3$ in the function of $m$ which is $2m^3-(3m^2+(4m-5)+m-9)$ we get a value of $26$ .