To start off, you want to create an equation. 18 subtracted six times from a certain number (let's use a variable 'x') would look like: 6x - 18.
6x - 18 = -42
Next, isolate the variable by adding 18 to both sides.
6x - 18 = -42
+ 18 + 18
6x = -24
Lastly, solve for x.
6x/6 = -24/6
x = -4