How do I get these rules from the sequences? Confused! Solve!

a.  This looks to be arithmetic.....common difference between terms =2

And the first term = 2

So

a_n  = 2 + 2(n - 1)   which we can simplify as   a_n = 2_n

b. Also arithmetic .....common difference = 2 ....first term = 3

a_n  = 3 + 2(n - 1)     which we can simplify as   a_n  = 2n + 1

c. Arithmetic....common difference = -25  and the first term is 50

a_n  = 50 - 25(n - 1)    =   75 - 25n  

4. Geometric.....common ratio  = 1 / (1/3)  = 3....first term = 1/3

a_n  = (1/3)*3^(n - 1)