How much money should the company spend on marketing each month?

The function represents the profit, and the company obviously wants to maximize it.

You can see that the profit increases until the parabola reaches its maximum.

The coordinate of the maximum look like (1.5, 90)

This means that, if the company invests 1.5 thousand dollars, the profit will be 90 thousand dollars, and this is the maximum profit the company can reach.

where's the diagrams?