It is easiest to see the intercept in the form where b is the y-intercept and m is the slope.

Use the point (x,y) and the slope (m) and plug it into the equation above. Plugging in y, x, and m will only leave you with b and you can solve for the y-intercept.

It's good to know that is called the "point-slope" form of a line..if my memory serves me correctly.