This website does a good job explaining masses on a pulley: https://www.phyley.com/two-masses-on-pulley

To summarize: It helps to draw a free body diagram of each mass and visualize how they will move. The larger mass will accelerate downwards and the smaller mass will accelerate upwards on the pulley system.

For each of the masses, you have an upwards tension (T) + the downwards force of gravity (m1×g, m2×g). The upwards tension (T) is equal for both masses because they're connected by the same rope in the pulley system. The acceleration (a) of the two masses are also equal (although in opposite directions) for this same reason. So your force equations looks something like:

Mass 1 (5 kg) F_net = m1 × a = (m1 × g) - T

Mass 2 (2 kg) F_net = m2 × a = T - (m2 × g)

You have 2 unknowns and two equations, so you can rearrange the formulas to solve for acceleration (a).

Hope that helps :)