Futility of Laying Pipe, Part 3

We’ve now analysed the original Steiner Tree Problem, turned it into a decision problem (SP), shown that SP is in NP (by finding a polynomial-time verifiable certificate), and laid out a polynomial-time reduction from 3-SAT to SP. Now, we need to prove our reduction correct in order to show SP is NP-hard. Together SP being in NP and NP-hard shows SP is NP-complete.