Write an explicit formula for the sequence 7,2,-3,-8,- then find the 14th term

The series of these numbers are arithmetic progression because every time you subtract the next term to the previous term, each difference is common (or constant), that 2 - 7 = -5, or -3 - 2 = -5.

You can use the formula of the arithmetic term, that is
an = a1 + (n - 1)d
where an = nth term, a1 = first term, n = number of terms, d = common difference

The 14th term will be
a(14th term) = 7 + (14 - 1)*(-5) = -58