solve {a_1*(x_2-x_1)^3+b_1*(x_2-x_1)+y_1=y_2,a_1*(x_2-x_1)*(x_2-x_3)^2+b_2*(x_2-x_3)+y_3=y_2,3*a_1*(x_2-x_1)^2+b_1=3*a_1*(x_2-x_1)*(x_2-x_3)+b_2} for {a_1,b_1,b_2}