Let $M\propto \left[F^a{L}^b{T}^c\right]$

Writing dimensions on both sides and using the principle of homogeneity of dimensions we have,

$\left[M^1{L}^0{T}^0\right]=k{\left[ML{T}^{-2}\right]}^a{\left[L\right]}^b{\left[T\right]}^c$

On comparing the power on both sides we get $a=1,a+b=0$ and $-2a+c=0$

On solving we have $b=-1,c=2,a=1$

$\therefore$ units of mass is $\left[F{L}^{-1}{T}^2\right]$