I just gave you all the necessary information. All it requires is plugging in the number given for the perimeter and the variable for the length of a side.
If this were a difficult problem I'd probably give you the answer, but I've given you everything you need.
Think of it this way:
- You have four sides, each part of the total perimeter
- You know that two of the sides are some number 's'
- By nature of being a square, all sides must be this number 's'
- So you have four sides of size 's', i.e. 4s (as you stated)
Now, think about what an equation is. It equates something on the left-hand side (LHS) to the right-hand side (RHS). You want to find 's', right? Well you know that the total perimeter is 4s, and you know the value of the total perimeter as 42. So what does the equation look like?