The equation of a straight line is y=mx+b, where m is the slope and b is the y intercept. Substitute your value for the slope to give you y=1/4x+b. Now substitute the complete coordinate into this equation and solve for b:
-3=(1/4×7)+b
-3=1.75+b
b=-4.75
So now you have the full equation, y=1/4x-4.75
Now substitute the incomplete coordinate so you can find h:
-1=(1/4×h)-4.75
3.75=1/4h
÷1/4 (which is 0.25 as a decimal)
h=15