Divide through by (4m-3). Then you have 2n+3=0. Solve for n, n=-2/3. Then divide the original equation by (2n+3). Then you have 4m-3=0. Solve for m, m=3/4. Thus m=3/4, n=-2/3, or both.
Did the problem give u the value of m and n? For Ex. say m=5, when a question is 4m it is like saying 4 x m which is 4 x 5 = 20. So another way of saying 4 times a variable (m and n) is like 4 times the number m and n represent.