Respuesta :

Use sum of arithmetic series:
Sum of n terms = (n/2)(2a + (n - 1)d)
where a is the first term, -6
d is the common diff, 6 - 2 = 4
and n is the number of terms, 20

Sum of first 20 terms
= (20/2)(2(-6) + 19(4))
= 640