[PRECALCULUS] Multiplying matrices Any help? I tried putting in on Desmos and my TI-83 calculator but I got error messages on both. Does that mean it's undefined or am I doing it wrong?

One cell phone plan charges $20 per month plus $0.15 per minute used. a second cell phone plan charges $35 per month plus $0.10 per minute used. write and solve an equation to find the number of minutes you must talk to have the same cost for both calling plans.