How to form a peak with a few Relus?

This may be a dumb question but how do you form a peak vertex (like a volcano) rather than a trough with a few Relus?

This is in reference to How does a neural net really work? | Kaggle from Lesson 3.

I tinkered with the interactive plotting tools to try to form a peak but can only form upward facing curves when I add the Relus together. If I change one of the Relus to be subtracted by the other I can form a peak but since the Relu cuts off at 0 and below I can’t just take the negative of that function to form a peak.

With that being the case how could you use Relus to approximately map to the positive values of a scatter plot that is best fit by a function like 1.5x^3 - 7x^2 + 4x + 12.


Would you subtract some but add others together?