The following equation represents a relationship between household consumption expenditure, c and income, y: c= 120+0.65y explain what the figure 120 stands for in the consumption income relationship, and the type of
relationship

120 is the MPC (marginal propensity consumption), and it has an independent relationship.

What is this supossed to be?

You're asking in the ap section, so i'll give the ap calculus-style answer. alternatively, you can find the vertex of the parabola and use the "y"-coordinate as the answer.

the maximum height can occur at a critical number, that is, a number  such that  .

the derivative of  can be calculated as follows:



find a value of  that is a zero of the derivative:


this is parabola that is opening downward; we can assume there is a maximum at  .

to find that maximum, calculate  .