Do you need $m$ ?

Discrete Mathematics Level 4

$(1+x)^2+(1+x)^3+(1+x)^4+\cdots+(1+x)^{49} + (1+mx)^{50}$

Let $m$ be the minimum positive integer such that the coefficient of $x^2$ in the expansion above is $(3n+1) \dbinom{51}{3}$ , where $n$ is an integer.