\(V_d = 15 mph\): Velocity downstream

\(V_u = 9 mph\): Velocity upstream

\(V_c \): Velocity of the current

\(V_s \): Velocity in still water

\(V_d = V_s + V_c\)

\(15 = V_s + V_c\)

\(V_u = V_s - V_c\)

\(9= V_s - V_c\)

By elimination eliminate \(V_c\)

\(24 = 2V_s\)

\(V_s = 12 mph\)

\(V_c = 15-12 = 3 mph\)