Since y = -3/4 is a horizontal line only passing through 0.7 on a graph, you need a positive line that starts at -8 on the y-axis and goes to 3 on the x-axis.
To find your answer, you have to remember in y=mx+b, m is your slope and more importantly, b is the y-intercept.
Since you need a line that goes through both 3 and -8, we already know the b should be -8.
Now all you have to do is find the slope, which is just your directions to get to 3. The coordinates are (0,-8), so to get from -8 to positive 3 you need to go up 8 over to the right 3.
So you should have y = 8/3x-8. Also, look at the graph above.