first step is to determine the slope of the line ( which is the rise over the run) or symbolically slope is defined as m= ∆x / ∆y, so plugging those values we get...
m= ∆x / ∆y = (-1 - 0) / (4 - 2) = -1 / 2
so next is to find the zero( y-intercept) of the function by ....