Question

The second term of an arithmetic sequence is 7. The sum of the first four terms of the sequence is 12. Find the first term, a, and the common difference, d, of the sequence.

Answer 1

T2 = 7
S4 = 12

7 = a + 6d
a = 7 - 6d

12 = 2 ( 2(7-6d) +3d )
6 = 14 - 12d + 3d
9d = 8
d = 8/9

a = 7 - 6(8/9)
a = 1/2/3

Answer 2

Are you sure its Arithmetic?  

There isn't a way to solve this, that sum of the first four terms has got me a little screwed up.

the formula you want to use is this:

An=A1+D(N-1)

A1 is the first term of the sequence D is a common difference and N is the term of the sequence.

7= 1 + 6 (2-1) 

that's the only way, so let's plug it in for the first one 3rd and 4th 

A1= 1 + 6 (1-1)
A1=1

A3= 1 + 6 (3-1)
A3=13

A4= 1 + 6 (4-1) 

A4=19


that's as far as I can help, though.