Hi there!
Great question! :D
We'll start by dividing both sides by 5.
It looks something like this:

So,

When in doubt of the answer, we substitute the variable for the 17 we got.
How?
Since a variable next to a number means to multiply, we'll set up our problem as a multiplication one.
5 Ă— 17 = 85
This is a TRUE statement.
Hope this helps! :D