Remember that we said when we encounter simultaneous equations stacked
up or side by side you need to focus completely on your target. The
target here is 3x + 2y. In looking at the two equations we can see
that subtracting 7x. from 10x would give us the value of 3x.. This
is the clue we’re looking for, so we subtract 7x – y = 9 from 10x + y =
8, we get a value of 3x + 2y = -1. B is the correct answer.