I’m assuming you’re trying to find the equation of a line that intersects the point. In order to do this you need to plug in your points into the equation that at the moment just has your slope.
y=5x —> 13=5(3)+b —> 13=15+b —> 13-15=b
—> b=-2
Now that you finally found your b (y-intercept) you can just plug it in!
y=5x-2