Are you sure you wrote the problem correctly?
If 4x = 5y = 4/3 and y = 3x -1 then:
4x = 4/3
x = 1/3
5y = 4/3
y = 4/15
y = 3x - 1
4/15 = 3(1/3) -1
4/15 = 1 - 1
4/15 = 0
This is not true, which means there is no solution to the problem as stated.
Should one of the = in "4x = 5y = 4/3" be a + or - ?