how do I calculate the 3rd term

Question

A new sequence is formed by adding together the corresponding terms of a geometric sequence and an arithmetic sequence. The geometric sequence has a common ratio of three and the arithmetic sequence has a new common difference of -2. The first two terms ,of the new sequence are 4 and 20

Answer 1

The geometric sequence: X1,3X1,____

The arithmetic sequence : X2,X2-2, ______

X1+X2=4

3X1+X2-2=20

Solving this system of two equations

X1=9, X2=-5

The arithmetic sequance: 9,27,81

the geometric: -5,-7,-9

The third term of the new sequence is 72.