Use slope intercept form for equation, which is y = mx +b. m is slope and b is the y-intercept.
Find slope by change in y over change in x. So pick two points, for example (0, 24) and (2, 20). Slope (m) will be
(y2-y1)/(x2-x1)
= (24-20)/(0-2)
= 4/-2
m = -2
Y-intercept is 24.
Equation is y = -2x + 24