Does the set of ordered pairs {(3, 11), (6, 1), (6, 4), (15, 4), (18, 18)} define a function?

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Margaret is purchasing a house for $210,000 with a 15 year fixed rate mortgage at 4.75% interest she has made a 5% down payment the house is valued at 205,000 and the local tax rate is 3.5% homeowners insurance 600 per year what are her total monthly payment

y=2x−1 5x−4y=1 is (1,1)(1,1) a solution of the system?