M = mass of the girl = 53.4 kg
m = mass of skateboard = 3.55 kg
V' = velocity of the combination of girl and skateboard before she jumps forward = 0
V = velocity of the girl forward = 1.32
v = velocity of the skateboard afterward = ?
Using conservation of momentum
(M + m) V' = MV + m v
inserting the values
(53.4 + 3.55) (0) = (53.4) (1.32) + (3.55) v
v = - 19.86