The first term is a1 = 18
The common difference is d=-4
And the nth term of an arithmetic sequence is given by
an = a1 + (n-1)d
Plugging in what we know:
an = 18 + (n-1)(-4)
So find the 18th term of this arithmetic sequence,
replace n with 18,
a18 = 18 + (18-1)(-4)
and simplify.