Assuming you meant, (3/4)x+(4/7)y = (5/4)y-1 and x=8
putting the value of x in (3/4)x+(4/7)y = (5/4)y-1 we get:
(3/4)8+(4/7)y = (5/4)y-1
or 6+(4/7)y = (5/4)y-1
or (5/4)y - (4/7)y= 7
or (19/28)y = 7
or y = 7*28/19
or y = 196/19
So the solution is x = 8 and y = 196/19