18,31a-a

I’m not sure what you’re asking exactly. If you want to simplify the expression "31a − a":

• Combine like terms: 31a − a = (31 − 1)a = 30a.
• So the simplified form is 30a.

If you meant something else (for example, problem 18 and problem 31a − a, or 31(a − a) which would be 0), please share the full statements and I’ll help step by step.