With these questions it's best to start by drawing in the forces of the body.
In this case, it has a Normal Reaction force which comes at right angles from the skiers motion, a weight force vertically down and a resistance force parallel to the motion (as its decelerating).
Then, you know that we need to find the resistance force, and to find this, we need to use the equation F=ma.
We need to resolve the weight force, to find its component that acts along the same line as the resistance force. (I don't really know how to explain this further, but see my attached diagram
The angle is the same as the angle given to us in question, so you use trigonometry to find the component. In this case its weight * sin(30). Let me know if you need me to explain this further.
Now we can insert these numbers into the equation of motion.
F = ma
wsin(30) - R = 70*-4
70*9.8*sin(30) - R = 70*-4
-R = 623
R(magnitude) = 623 N
You need to make sure you have assigned the negative signs correctly.
Hope this is helpful (also I apologise if my answer is incorrect, its been a while since I've done school work oops. The theory is correct though.) Let me know if you have anymore questions!