sorry what I meant is don't you need the weight to work out the moments of the support idk
so the rod is uniform, meaning the weight of the ruler is acting from the centre. Draw a diagram with the two supports, and their forces (which go upwards). Then take moments and solve for the upward force from each support.
N.B. the total moments cw = 0 and you will need to take moments about one of the supports to eliminate one variable from the equation and make it solvable
because it states the ruler is uniform you know its weight is going to act through the centre (500mm). use this to set up a diagram of the moments representing the knives as reaction forces at 100mm and 200mm. you can then use the principle of moments to generate two equations where the two reaction forces (i call them R1 and R2) are unknown. rearrange the equations into the form R1= .... then equate them to eliminate R1. you can then solve for R2. once you’ve found R2 you know the system is in equilibrium to therefore the sum of the forces acting down (weight of ruler) is equal to the sum of forces acting up (R1 and R2). this means R1 R2 = 1.2 therefore R1 =1.2 - R2.
sorry this was a mess of an explanation but i’ll attach a picture of my working to hopefully make it easier to understand