Monopoly: Demand Curve and Marginal Revenue Curve Intercepts
Your marginal revenue is not calculated correctly. Marginal revenue $(MR)$ is the derivate of total revenue which is equal price times quantity $TR=PQ$.
In your case $TR$ should be:
$$TR=(k+aQ)Q \implies MR = \frac{dTR}{dQ} = k+2aQ$$
If the demand is given as: $P = 120−2Q$ then:
$$TR= (120-2Q)Q \implies MR = \frac{dTR}{dQ} = 120-4Q $$
Also made a graph for you by simulating the $MR$ (red) and demand (blue) in R