Answer
$T\gt w_{1-α}$: null hypothesis is rejected.
There is enough evidence to conclude that $M_x\gt M_y$.
Work Step by Step
Right-tailed test.
Small-sample case:
$T=S-\frac{n_1(n_1+1)}{2}=220-\frac{12(12+1)}{2}=220-78=142$
$w_α=w_{0.05}=56$
(According to table XIII, for $n_1=12$, $n_2=15$ and $α=0.05$)
Critical value:
$w_{1-α}=n_1n_2-w_α$
$w_{0.95}=12\times15-56=124$
Since $T\gt w_{1-α}$, we reject the null hypothesis.
