I need help with my maths

What is the quadric regression equation that fits these data

A33 gram sample of a substance that's used to detect explosives has a k-value of 0.1473. n = noekt no = initial mass (at time t = 0) n = mass at time t k = a positive constant that depends on the substance itself and on the units used to measure time t = time, in days

If t17 = 3 (t5) in an arithmetic progression, find t1 in terms of d.