Since you're looking for the x coordinate, and already have the y coordinate, you can just plug in the y coordinate and then solve for x.
If you plug in -1 for y, then you get this:
3x - 4(-1) = 11
Then just solve.
3x + 4 = 11
3x = 7
/3 /3
x = 7/3
So the missing coordinate is 7/3.
