the sum of 6 and 7, plus 5 times a number, is -12
The sum of 6 and 7 means we add the two numbers:
6 + 7
This evaluates to 13
Next, we take 5 times a number. The phrase a number means an arbitrary variable, let's call it x. So we multiply x by 5:
5x
The first two words say the sum, so we add...