First, you have to convert the equation into slope-intercept form. (A.K.A. get y alone on one side of the equation)
7x - 4y = 8
-7x -7x
-4y = -7x + 8
---- ----------
-4 -4
y = -7/4x - 2
Now we can plug in 0 for x.
-7/4 * 0 is 0
0 - 2 is -2
The y-intercept is -2.