If, y = mx + c , is the equation of a straight line, where m=gradient (i.e. the slope), and c=y-intercept.
y = -x/2 + c
The x-intercept is when the line intercepts thex-axis, i.e. when y=0 and x=4. Put this into equation.
So, 0 = -4/2 + c
0 = -2 + c
c = 2
So, the equation becomes:
y = -x/2 + 2