a1 = -4, means the first term of the sequaence = -4
In the arithmetic sequence, there is a common difference between each two consecutive terms
We can find this difference from the given rule
an = a n-1 + 11
an-1 is the previous term of an
11 is the common difference
The rule of the sequence is
a is the first term
d is the common difference
n is the position of the term
Substitute a by -4
d by 11
Multiply the bracket by 11
Add the like terms -4 and -11
The rule of the sequence is a(n) = 11n - 15