From what radioactive element does fluorine-19 form by positron emission?

A positron is a particle with a mass of zero and a charge
of +1.  So to solve, you write a balanced equation so that the atomic numbers and total
mass are equal on both sides of the equation.

Element X
--> F-19 + positron (mass = 0, charge = +1)

F-19 has
an atomic number of +9.

Add the +9 to the +1 for the
positron means the unknown element has an atomic number of +10 which is
Neon.

The isotope of neon would be
Ne-19.