find the equation of a line with slope 4/5 and y-intercept (0,-7) Do I still use the formula y=mx+b?
Yes. Let me know what you come up with and I'll let you know if you are correct.
So far I came up with
y+7 = 4/5(x-0)
Now I am stuck...
OK, you could do it that way, and distribute the 4/5 and subtract the 7:
y=4/5x-0-7
y=4/5x-7.
Or, you could just start with y=mx+b. If you didn't notice, the point they gave you was the y-intercept because the x-value was 0, meaning that was where the line crosses the y-axis. So, since b is the y-intercept, you could have just immediately written
y=4/5x-7