You could use your 3 points and find 3 equations with 3 variables to substitute back into the general quadratic model

With (0. 0), (1) ,c = 0.With (151, 5), (2)

With (686, 30), (3)

Solve the system of equations (2) and (3) foraandb. Then plug them back into the general formula for the quadratic.c = 0, so the equation will have only a quadratic term and a linear term.