What is the equivalent resistance for a parallel circuit that has two resistors: 18.0 ohms and 23.5 ohms?

... take the reciprocal of each resistor.
... add the fractions
... take the reciprocal of their sum; that's the answer.

When there are only two resistors, here's an easier way:

... find their product
... find their sum
... divide the product by the sum; that's the answer.

Your parallel resistors make 10.193 ohms.