Rewrite the sum of 81 and 36 by factoring out the greatest common factor

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

Can someone me out here and the tell me the greatest common factor

An investment decreases in value by 30% in the first year and decreases by 40% in the second year. what is the percent decrease in percentage in the investment after two years