How do you solve this question. I literally have no idea on where to start. "If the mean of the following numbers is /7, find the c value. Produce an algebraic solution.

Question

How do you solve this question. I literally have no idea on where to start. "If the mean of the following numbers is /7, find the c value. Produce an algebraic solution. 
Numbers: 12, 18, 21, c, 13

Please help.

Sandy · Answer

Sorry. There's no / before 7. A typo. It's actually 17

Ms. Sue · Answer

(12 + 18 + 21 + c + 13) / 5 = 17 64 + c = 85 c = 21