If x=16-4y then in your other equation 3x+4y=8 you can put that where x is to get:
3(16-4y)+4y=8
Then multiply brackets to get:
48-12y+4y=8
Simplify:
48-8y=8
Rearrange:
40=8y
Divide through by 8:
y=5
Then put y=5 back into x=16-4y to get x:
x=16-4(5)
x=16-20
x=-4
