Since we know the slope is 3 we have part of the equation already (y=3x+b). Now we need to find the y-intercept. To do that we plug in the point that was given to us in the question and solve for b.
0=3(-5)+b
0=-15+b
15=b
Now that we have the y intercept we can put the equation together.
Answer: y=3x+15
Hope this helps :)