Additive model ANOVA with interaction. How to obtain gammas from the graph?

Question

I try to complete this exercise of statistic that said that I need to compute the values of alpha, beta and gammas from the graph.

Graph:

I allready compute the values algebricaly, and this are:

alpha 1: 0.5
alpha 2: -0.5
beta 1: 0.5
beta 2: -0.5
gamma 11: -1.5
gamma 12: 1.5
gamma 21: 1.5
gamma 22: -1.5

I also obtain grapically the alphas and betas for the given graph. But my question is: How can I pick the values of gammas from the graph?

Below are the equations for the additive model:

Thanks for your help guys.