1.28 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam

Clockwise moments = anticlockwise moments
Moment = force x perpendicular distance from pivot
Think.
If the the distance from the pivot is less on the left hand side it means that the force must be greater to compensate for the larger distance on the right hand side.

The upward reaction force on the left (RA) is greater than that on the right (RB). 

Taking moments about A gives: 

600 × 2.0 = RB × 6.0 → RB = 200N 

RA = 400N because RA + RB = 600N 

(Or you could take moments about B)