what is the 20th term of -6,-4-,-2
This is not calculus.
This is Arithmetic Progression, where:
a1 = the initial term = - 6
d = the common difference of successive members = 2
an = the nth term
an = a1 + ( n - 1 ) d
a20 = a1 + ( 20 - 1 ) d
a20 = a1 + 19 d
a20 = - 6 + 19 ∙ 2 = - 6 + 38
a2 = 32