Firstly, find your base. The cost of Monica's swimsuit is your base here. (35)
Next, find Monica's mother's swimsuit cost. (35 x 2 = 70)
After that, add the total cost of those two swimsuits. (105)
Finally, figure out what you have to add to the lesser side to make it even with the higher side. (170 = 105 + 65)
Monica's father's swimsuit would cost $65.
For reference, your equation could look like:
35 + 35 x 2 + X = 170
