When you're solving an equation for a variable (in this case x) you are trying to get x on one side of the equal sign all by itself. To do that, you do the opposite of what is being done to it (so that you can get the number in front of the x to equal 1). So when you have:
17 x = -85
x is multiplied by 17 -- to get rid of that 17, we have to divide by 17 on both sides of the equation. You have to do it on both sides of the equation to keep the equation balanced.
17 x ÷ 17 = -85 ÷ 17
On the left side of the equation the 17s get canceled out (specifically because 17/17 = 1), so what you're left with is:
x = -85 ÷ 17
That you can punch into a calculator to get
x = -5