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The answer is 12. Substitute a for 12 and b for 7 in the equation. Multiply first, then add/subtract from left to right
gdhillesheim

First you would replace "b" with "7" and then "a" with "12"

You would have 3(7) - 2(12) - 9

Next step would be combining 3 and 7, also combine 2 and 12

You would have 21 - 2 x 12 - 9

Multiply 2 and 12 (24) and put back in equation

Then you would end up with 21 - 24 - 9

Subtract all that and you end up with -12

Answer: -12