Could it be a proportional function?

Explain step-by-step how to simplify -5(2x – 3y + 6z – 10).

Look at attachment. has all info needed. need

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is