Someone plz help me

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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

Can someone me and do the problem plz so i can understand it more better

Describe the 2 algebraic methods you can use to find the zeros of the function f(t)=-16t^2+400.