The question is asking you to solve for A
to determine the first, fourth and tenth term you simply put those values in for (n)
the first term you then be given by the equation A(1) = 12 + ((1)-1)(3)
A= 12
Same thing for 4th term A(4) = 12 + ((4)-1)(3)
A= 21
10th term: A(10) = 12 + ((10)-1)(3)
A= 39
∴ your answer is C