help anyone please i need it

Answer:
The sum is equal to [tex]-905,200[/tex]
Step-by-step explanation:
we know that
The formula to calculate the sum is equal to
[tex]Sn=n\frac{(a1+an)}{2}[/tex]
In this problem
[tex]n=775[/tex] -----> number of terms
a1 is the first term
[tex]a1=(-3(1)-4)=-7[/tex]
an is the last term
[tex]an=(-3(775)-4)=-2,329[/tex]
substitute
[tex]Sn=775\frac{(-7-2,329)}{2}=-905,200[/tex]