To find explicit formulas, you need two things. The common difference and the first term.
For example, #18
The first term = -15
The common difference = -20 -(-15) = -20 + 15 = -5
y = A + B(n - 1)
A = our first term
B = our common difference
n = the term you want to find in the sequence.
Leta plug our numbers in from #18
y = -15 + -5(n - 1)
Let's find the 4th term in the sequence.
y = -15 + -5(4 - 1)
y = -15 + -5(3)
y = -15 + -15
y = -30