So basically, what they're saying is that m is between 6 and 12. You can also write that as 6 < m < 12.
What we need to do is solve two different equations for the cost, one with m as 6 and one with m as 12, to get the range of the cost.
When we solve C = 2(6) + 8, we get $20
When we solve C = 2(12) + 8, we get $32
So the range would be between $20 and $32, which you could also write as 20 < c < 32.
Hope that helps!