You have the following sequence:
6, 18, 54, 162, ...
You can notice that each element (except the first one) is three times the previous one. Then, for the recursive rule you have:
a) recursive rule:
![a_n=3\cdot a_(n-1)](https://img.qammunity.org/2023/formulas/mathematics/college/t8bltt57ybr2tqvyx4lin2uw4k0d6dws1b.png)
Based on the previous result, you can conclude for the iterative rule:
b) iterative rule:
![a_n=6\cdot3^(n-1)](https://img.qammunity.org/2023/formulas/mathematics/college/emgmxo3w3892t95tt9gm5go8kjhbpdcta2.png)
If you replace n by the index of each element you obtain the given sequence.