Hoi!
Here's how you would write this.
z - 34.7 = 68.2
"Less than" indicates subtraction, but from what? "Less than z", so we're subtracting something from z. If you look at the equation, how would we calculate 34.7 less than z? To do this, we would subtract 34.7 from z. Since we're given our answer to the equation, just slap that into the equation after an "equals" symbol, and you're finished!