The very first thing to do in this problem is figure out how to "describe" how many markers Dave has!
Let's call Dave's "unknown" number of markers "x".
If Sheila has five times the number of markers that Dave has, then Sheila must have "5 times x" -- or (another way you could write that would be) "5x".
You are now close to solving the problem!
The last step to solving this problem is to combine the "unknowns" on one side of the =, and the "knowns" on the other side side, like this:
5x (those are Sheila's markers) + x (that represents Dave's markers) = 18
What's do you get when you combine 5x and 1x ("x")? Of course: 6x.
6x = 18
I bet you already know what to do now! But in case you're still "stuck"...
---> Now you need to know what x is!
Remember the rule for solving this kind of prolem: "What you do to one side, do to the other side"?
So if you want to divide 6x by 6 to solve for x, then you must also divide the other side by 6, too! Like this:
6x/6 = 18/6
Wow! Now that made everything sooo clear:
6x/6 gives you x, and 18/6 gives you 3.
Finally, the "proof of the pudding is in the tasting"! The way to "taste-test" your answers is by "plugging in" your answers to the original problem:
6x = 18
6 (3) =? 18
YES! 6 times 3 is 18!
p.s. Hope this helps! And btw, you can use these same steps next time when you get a problems like this one!