y=-18 sin 3.1415/6(x-1) = 43.5

y=43.5

y=-18(sin(pi/6))(x-1)

sin(pi/6)=0,5 -----> 43,5=-9(x-1)

                               43,5=-9x+9
                               43,5-9=-9x
                                       x=34,5/9=3.8

I´m not sure if I did the equation as you wanted it or if you ment y=-18sin((pi/6)*(x-1))