How do you put this into an equation. full tank of gas is 525 miles. only driving round trip to work takes 13 miles a day. how many days can i drive before i have to fill up again. let y=miles to and from work. x= miles to next fill up. d= days i can drive

Watch for units in questions. This will help you set up equations. You can do math with units. That's why area is in units of distance squared, (distance × distance  = distance²) etc.

There is a rate given (units/units) which is 13 miles/day. When you multiply this by days, the days cancel out by division leaving miles. Or if total miles is divided by the rate (flip & multiply for fractions) you get days.

525 mi × (1 day / 13 mi) = days you can drive until fill up
~ 40.4 days

** at the end of 40th day or maybe even beginning of the 41st you will prob need to fill up the gas tank.

The problem wants you to replace these quantities with variables.
y = miles to/from work (13 mi)
x = miles until fill up (525 mi)
d = days you can drive

Then just replace the quantities with the assigned variables...
x/y = d