How do you find the equation of a line that has a x intercept of 5 and a slope of 4?

I assume you mean the slope-intercept formula, y=mx+b

First, put it in point-slope form. that is y-0=4(x-5)

the 4 is the slope, and the 0 and five are from the x-intercept

y-0=4(x-5) simplifies to y=4x-20, which is the equation you wanted