Let s = soft shell tacos, and h = hard shell tacos
6s + 2h = 12
3s + 1h = 6
Im not sure you can even write a system for this since they are proportionate.
(3s + 1h = 6) * 2 --> 6s + 2h = 12, which is the first sentence. If you wrote a system, it would just be "0 + 0 = 0"; you can't cancel anything out.
However, you could make a good guess to what the answer might be since the numbers aren't big.
s could equal 1 and h could equal 3.