Working out reaction forces in momentsWatch
Okay, so the first one, take the moments about point A and assume the weight (a force) of the beam acts downwards at its center.
Then assuming the beam is in equilibrium and not rotating, take moments about point A with clockwise being positive. You have:
The numbers in brackets are the distances in meters and both 200 and C are forces with the unit N. C must therefore be a force in units and you get 125N.