a Find the sum of the geometric sequence round answer to two

a) Find the sum of the geometric sequence. round answer to two decimal places if necessary

6 - 18 + 54 -162 +486 - ... + 6*(-3)^13

The answer is -7,174,452 but I cannot figure out the right way to get to this answer

b) Find the sum

-3+1+5+9+13+...+(4n-7)

answer: 2n^2-5n

c) Find the indicated term using the given information

-14, -19, -24, ...; a47

answer: -244

1) nth term is 6*(-3)^13 = -9565938

An = a1 * r^(n-1)

-9565938 = 6 * (-3)^(n-1)

n = 14th term

hence we have to find sum of first 14 terms

S14 = a1 ( 1 - r^n ) / ( 1- r)

r = -3

6 * ( 1 - (-3)^14 ) / ( 1 - (-3)

S14 = -7174452

2) -3 + 1 + 5 +9 + 13 +...+ (4n - 7)

it is an arithmetic sequence

common differernce d = 4

a1 = -3

sum = n/2 [ 2a1 + (n-1) d ]

= n/2 [ 2(-3) + (n-1) 4 ]

= n/2 [ -6 + 4n - 4 ]

n/2 [ 4n - 10 ]

2n^2 - 5

c) -14. -19 . -24 , ...a47

a47 = a1 + (n-1)d

= -14 + (47-1 ) -5

= -244