Answer
see below.
Work Step by Step
The sum of the first $n$ terms of an arithmetic sequence can be obtained by the following formula: $\frac{n(a_1+a_n)}{2},$ where $a_1$ is the first term, $a_n$ is the nth term and $n$ is the number of terms.
The nth term of an arithmetic sequence can be obtained by the following formula: $a_n=a_1+(n-1)d$, where $a_1$ is the first term and $d$ is the common difference.
Hence here: $a_1=-3,d=1-(-3)=4$, thus $a_{33}=-3+(33-1)4=-3+128=125.$
$a_{20}=-3+(20-1)4=-3+76=73.$
Thus the sum:$S_{20}=\frac{20(-3+73)}{2}=700$