The third term in a sequence is 11.
The term-to-term rule is "take away 4".
Write an expression, in terms of n, for the nth term of the sequence.

a(n) =  a(n - 1) - 4, with a(1) = 19  

Step-by-step explanation:

If the rule for finding each new term is "take away 4," then this is an arithmetic sequence with first term a(1) (unknown) and third term 11.  The "common difference" is -4.

Then the formula for this arithmetic sequence is found as follows:

The third term is 11.  The previous (second) term is 4 greater, or 15.  The first term (coming before 15) is 19.

Thus, the general formula is

a(n) =  a(n - 1) - 4, with a(1) = 19  

Check:  Does a(1) come out to 19?  Is 19 - 4(1 - 1) = 19?  YES

Does a(2) come out to 15?  Is 19 - 4 = 15?  YES

Does a(3) come out to 11?  Is 15 - 4 = 11?  YES

b.an increase in a causes b to decrease