I am assuming that you mean the slope equals 4/5.
We can put the equation into y=mx +b form.
m is the slope, so we already know that m = 4/5
Next, we have to find b.
If we have the point (1, -1) and we know that the slope is 4/5, then we can just subtract 4/5 from -1 to get our y-intercept.
So, the answer is....
y = 4/5 x - 9/5