How do I find the equation of a line given the x-intercept

ok, so here is my question:
find the y-mx+b form of a line passing through points (3, -1) if:
a)the x intercept is (-1, 0)

i know if there is an x-intercept the y value is always 0, but i have to solve it without having 2 variables, the only variable should be m or b, how do i solve this? please explain the method, not just the answer, thanks!

Take two points and substitute 'em for the slope formula!

m = (y - y1)/(x - x1)
m = (-1 - 0)/(3 - (-1))
= -¼

Finally, substitute (-1,0) and the slope of -¼ for y - y1 = m(x - x1):

y - 0 = -¼(x - (-1))
y = -¼(x + 1)
y = -x/4 - ¼

I hope this helps!