Answer:

Step-by-step explanation: 
<u>Key skills needed: Explicit formula, Geometric sequence</u>
1) This is a geometric sequence, meaning a constant (like 6 or 3.5) is being multiplied to every term. First we need to find the constant. 
This can be done by doing -42 ÷ 7   or   252 ÷ -42 
-42 ÷ 7 equals -6. 
252 ÷ -42 also equals -6. 
2) This means that constant is -6. 
3) The general form for an explicit formula of a geometric sequence is: 
 
 
--> r means the constant (or common ratio) that is being multiplied. 
-->  means the first term
 means the first term
-->  means the "nth" numbered term (so
 means the "nth" numbered term (so  is the 3rd term,
 is the 3rd term,  is the 10th term and so on).
 is the 10th term and so on). 
-->  is just the "n" in
 is just the "n" in  minus 1.
 minus 1. 
4) We can use this formula to create an explicit formula for the geometric sequence in the problem. 
-->  (Since we got -6 as the constant/common ratio).
 (Since we got -6 as the constant/common ratio). 
-->  (Since 7 is the 1st term)
 (Since 7 is the 1st term)
This means that our formula would be --> 
<em>Hope you understood and have a nice day!! :D</em>