This language is NOT CFL. It is given that m is a multiple of n, so m = n*k where k is integral. But then, the value of k is ambiguous. It can be any integer ranging from 0 to +inf. If the value of k was given, suppose k = 4; then

$a^{n}b^{4n}$ would have been CFL.

$a^{n}b^{4n}$ would have been CFL.