What is the solution a comma b to this system of linear equations? 3 a plus 6 b is equal to 45; 2 a minus 2 b is equal to negative 12

Divide the first equation, 3a + 6b = 45, by 3 to get a + 2b = 15. Divide the second equation, 2a − 2b = −12, by 2 to get a − b = −6, so a = b − 6. Substituting gives (b − 6) + 2b = 15, hence b = 7 and a = 1. The solution is 1 comma 7, choice D.

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