Could someone help me with this, please, I’ve been struggling for the past few hours, and I need to finish this within the next hour or I’ll get a zero, please anyone, and thank you if you do know how to help me

Answer:
-383.5
Step-by-step explanation:
4+2.5+1+...-33.5
a=4
c.d. d=2.5-4=-1.5
l=-33.5
l=a+(n-1)d
-33.5=4+(n-1)(-1.5)
-33.5-4=(n-1)(-1.5)
-37.5=(n-1)(-1.5)
[tex]n-1=(-37.5)/(-1.5)=25\\n=25+1=26\\n=26\\S_{26}=\frac{n}{2} (a+l)\\=\frac{26}{2} (4-33.5)\\=13(-29.5)\\=-383.5[/tex]