This might help it has tons of different problems for your subject.
https://www.wyzant.com/resources/lessons/math/algebra/calculators/proportion
Given that the First term of the Arithmetic sequence = -3
⇒ a = -3
We know that nth term of a Arithmetic sequence is : Tn = a + (n - 1)d
where a is the first term and d is the common difference
Given that 10th term is 15
⇒ T₁₀ = -3 + (10 - 1)d
⇒ -3 + (10 - 1)d = 15
⇒ 9d = 18
⇒ d = 2
⇒ Common difference of given Arithmetic Sequence is 2
I^2 = -1
so,
-99 = -1*99 = i^2*99 = 99i^2
