Looks like you just evaluated the summand for the given value of \(n\), whereas the question is asking you to find the value of the sum for the first \(n\) terms.
Let \(S_k=\displaystyle\sum_{n=1}^k\frac3{(-2)^n}\). Then \(S_k\) is the \(k\)th partial sum.
\(S_1\) happens to be the first term in the series, which is why that box is marked correct:
\(S_1=\displaystyle\sum_{n=1}^1\frac3{(-2)^n}=\frac3{(-2)^1}=-1.5\)
But the next partial sum is not correct:
\(S_2=\displaystyle\sum_{n=1}^2\frac3{(-2)^n}=\frac3{(-2)^1}+\frac3{(-2)^2}=-0.75\)
and this is not the same notion as the second term (which indeed is 0.75) in the series.
