1) Impulse: 
2) Strong force: -131.1 N, weak force: -44.7 N
Explanation:
1)
The impulse exerted on an object is equal to the change in momentum of the object itself.
Mathematically:

where
m is the mass of the object
u is the initial velocity
v is the final velocity
For the skateboarder in this problem, we have:
m = 53.6 kg
u = 4.45 m/s
v = 0 (it comes to a stop)
Therefore, the impulse is

Where the negative sign indicates that the direction is opposite to the motion of the object.
2)
The impulse is also equal to the product between the force applied and the duration of the collision:

where
I is the impulse
F is the average force
is the time during which the force is applied
The strong force is applied in a time of

Therefore this force is

The weak force is applied in a time of

So this force is

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