First you must find the y intercept, which is found by setting your x co-ordinate to zero in the equation. This equation already has x as a zero, so your y intercept is (0, -5). Now you must find the slope, or (m). m is given to you as 4/5, so m=4/5. At this point you simply need to make it slope-intercept form, where the coefficient of x is the slope, and the y intercept is the number added ( or subtracted ) to/from it. That is to say, (y = mx + b). Now all you need to do is plug in your y co-ordinate at the intercept at b and your slope at m. (x and y are your variables, so you don't need to worry about them).
(y = 4/5 - 5) is the answer
I hope I have clearly explained how to express an equation in slope intercept form.