We know that 3/10*20=6 students are eating salads.
We also know that 3/5*20=12 students are eating sandwiches.
Assuming each person only eats one thing, there are 20-12-6=2 students eating lunches other than salads or sandwiches.
We have:
ba + 3ca = -2
Extract common factor "a":
a(b + 3c) = -2
Divide by the term (b + 3c):
