Solving with delta

I'm doing Microeconomics. Sometimes i need to solve with delta, but when im trying to solve with delta on PTC, it doesn't work.Im sure, im setting the calculation up right, but i just don't know how to use the deltas right. My classmate that uses a program called wolframe alpha, and it works when she's setting it up the same way on that program. I'll stay updated here, so if you got any further questions, ill do my best to answer you guys
It's about profit maximizing and setting marginal revenue equal to marginal cost.


