0.24x+16.4=-1.48 what’s the answer help

Given the following statements, which is the best

What are “like terms”? why can we only add like terms?

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