Stu M. answered 12/10/20
Math Minor, Six Years Experience as a Full Time SAT Prep Tutor
Isn't it odd that this question is asking for 3x + 3y?
Why not x? Why not y? Why do you think the test-makers chose to ask for that specific value?
We might think to solve for either x or y and then use substitution to solve the system, but this takes a while...
7x - 5y = 4
4x - 8y = 9
From the first equation,
7x = 4 + 5y
x = (5/7)y + 4/7
And when we plug it into the second equation...
4 [ (5/7)y + 4/7 ] - 8y = 9
This is UGLY and difficult to deal with. Yikes.
We also might think to solve this system by elimination, i.e. multiplying both equations by a constant so that we can eliminate a term...
[ 7x - 5y = 4 ] * 4
[ 4x - 8y = 9 ] * 7
28x - 20y = 16
28x - 56y = 63
This is also UGLY and difficult to deal with. Yikes again.
There's a hint in the question that points to a shortcut. The problem is asking for the value of 3x + 3y, so we should look for a way to combine the equations to find this value.
Notice that when we subtract the two equations, we get the quantity 3x + 3y:
[ 7x - 5y = 4 ]
- [ 4x - 8y = 9 ]
--------------------
3x + 3y = -5
7x - 4x = 3x, -5y - (-8y) = 3y, and 4 - 9 = -5
Therefore, by simple subtraction, we have found that 3x + 3y = -5 and the answer is B.
Look for weird values in the question and unique ways to combine the equations in order to get to the problem faster on these system problems.

Brenda D.
12/10/20