a1 is the 1st term
r is the difference, they can say "r" or "d" for the difference
1st a1 is the first term = -3
2nd "r" is the difference between each consecutive terms (coming after each other) = 1.5
the rule to get any term of the sequence as we add 1.5 each time is --> a1+(n-1)*d
a1 --> 1st term , n --> order of the term u want whether 2nd, 3rd , 25th
d --> difference
so to get the 2nd--> -3+(2-1)*1.5= -1.5
to get the 3rd --> -3 +(3-1)*1.5 = 0
and so on
