If - MX ˇ NY is a monomial about X and y, and the coefficient is 3 and the degree is 4, then M =? N =?

If - MX ˇ NY is a monomial about X and y, and the coefficient is 3 and the degree is 4, then M =? N =?

It can be concluded from the questions:
-m=3 n+1=4
Then M = - 3, n = 3