As we know the property of charge that charge is always conserved
so we can say
[tex]Q = q_1 + q_2[/tex]
here we know that total charge is
[tex]Q = -8 C[/tex]
and we also know that
[tex]q_1 = -2C[/tex]
from above equation we can say
[tex]-8 = -2 + q_2[/tex]
[tex]q_2 = -8 +2 = -6 C[/tex]
so the other charge will be -6 C