What is the explicit formula for this sequence? -7, -3, 1, 5

We have the following sequence:
-7, -3, 1, 5,

From the sequence we get,
a = a1 = -7
a2 = -3
a3 = 1
a4 = 5

Now,
a2-a1 = 4
a3-a2 = 4
a4-a3 = 4

From this we know that,
=> an+1 - an = 4 (the common difference d)

Hence the formula for this sequence is
an = a + (n-1)d
=> an = -7 + (n-1)4

Uneed to add them up 27+18+23=68

What the heck don’t understand