I find different things online so I ask the gurus here. How do you do the math to figure out how many watts an amp/receiver will put out at a certain impedance. I realize this only gives you a rough figure but good enough for me and I want to know how to calculate it correctly.

Let's say you know piece of equipment X will put out 90x2@8 ohms and 120x2@4 ohms. What will this same piece of equipment put out at 30 ohms?