To add on to Wiggin42's answer (which is correct), I would like to take the time to explain the reason why this TA multiplied 2 on both sides of the equation. Wiggin42 multiplied 2 on both sides because in addition to the the given standard form formula:
There are also 3 rules that should be followed accordingly:
1. A shouldn't be negative.
2. A and B shouldn't both be zero.
3. And, A, B, and C should be integers.
#3 explains this reason. Because, before the problem was multiplied by 2, there was fractions. And, an integer is defined as a whole number.
Examples of an integer would be -2, -1, 0, 1, 2, etc.
Standard form is given by the formula:
Ax + By = C
where A, B, C are constants.
Multiply by 2 to get:
x + y = 12
Therefore, A = 1, B = 1, C = 12