His route from school to Rhianna's house to his house forms a triangle in which the sides are 7 miles and the hypotenuse is unknown. Solving using the Pythagorean theorem, we find the route from Rhianna's house back home is 7^2+7^2=x^2.
Rounding to the nearest tenth, we now know this route is 9.9 miles. Adding the total distances from each location, we get an answer of 7 + 7 + 9.9 = 23.9.
When dividing rational numbers, multiply by the "right" reciprocal. In this case, the "right" reciprocal means to take the reciprocal of the fraction on the right-hand side of the division operator.
