okay, we're not integrating with respect to x, or they would have asked for F(x). there are no special rules for this one. the derivative of tan(x) is sec^2(x), so just go backwards when integrating:

f(t) = 3sec^2(t) - 10t^3

=> F(t) = int{3sec^2(t) - 10t^3}dt

...........= 3tan(t) - (5/2)t^4 + C

Now F(0) = 0

=> 0 = 3tan(0) - (5/2)(0)^4 + C

=> C = 0

so F(t) = 3tan(t) - (5/2)t^4

so i guess you were integrating with respect to x and treating the function of t as a constant....not correct i'm afraid. if they wanted you to integrate with respect to x, they would let you know explicitly