Given both intercepts, you have two points,
if the y-intercept = -3, then you know that one point on the line is (0,-3)
if the x-intercept = 5, then you konw that the other point is (5,0)
Now all you need is the point slope form of the line:
All you have to do is decide which point will be 1 and which will be 2 (it
doesn't matter which is which as long as you are consistent), make
the substitutions, and do the arithmetic. To put your result into
slope-intercept form, solve the resulting equation for y.
I'll let you do the substitutions and the arithmetic.