How to find the x intercept in slope intercept form

-- The x-intercept is the point where the line crosses the x-axis.
-- 'Y' is zero at every point on the x-axis. 
-- So  y=0  at the x-intercept.

You say that you have the equation of the line, and it's in
slope/intercept form ?
                                      Y = mx + b
 Great !  

Just set  y=0.   Then you have      mx + b = 0 .
Solve that for 'x' , and you have the x-intercept.

(It turns out to be    x = - b/m .  Always true, and a very handy formula
to remember if you can, and then you always have the x-intercept at
your fingertips whenever you need it.)