Help please!!! i don't understand

Which is an equation for the nth terms of the sequence 12,15,18,21

Apple pie is removed from 350 degree oven. temperature of the pie t minutes after being removed id given by the function t(t)=72+268e-0.068t

Use the quadratic formula to solve the equation. if necessary, round to the nearest hundredth. x^2 - 20 = x a. 5, 4 b. -5, -4 c. -5, 4 d. 5, -4