u_y=0 when the projectile is launched horizontally. You can find s_y through the following methods.
1. Use s_y=u_yt+\frac{1}{2}(-g)t^2, since u_y=0 we can simplify the expression to s_y=-\frac{1}{2}gt^2
2. Use v_y^2=u_y^2+2(-g)s_y, which can be rearranged to s_y=-\frac{v_y^2}{2g}