Is this 7x+2y? If so, we can write by substitution: -7*5/6+2*3/8. 2 divides into 8: -7*5/6+3/4. The lowest common denominator of 6 and 4 is 12; 6 goes into 12 2 times; 4 goes 3 times. So 5/6 is 5*2/12 and 3/4 is 3*3/12. Put these values in the expression: -7*10/12+9/12=(-70+9)/12=-61/12 (-70+9 is the same as 9-70). You can divide 12 into 61 to give 5 and 1 over, which is 1/12, but remember it's negative so we have minus 5 and a twelfth.
If the question is 7x+y^2, the answer is different. y^2 is 3/8*3/8=9/64. The lowest common denominator of 6 and 64 is 3*64=192. 64 goes into 192 3 times and 6 goes 32 times. The expression becomes -7*5*32/192 + 9*3/192=(-1120+27)/192=-1093/192. This is minus 5 and 133/192.