Source: http://ie.lbl.gov/education/glossary/AnimatedDecays/BetaDecay+.html

In positron emission a proton in the nucleus is transformed into a neutron and a positron is ejected. This causes the atomic number (Z) to decrease by one while the mass number (A) remains the same. This is the same net result that would occur during electron capture.

In the example shown above an isotope of fluorine undergoes positron decay and becomes an isotope of oxygen.

The process can be represented in a balanced nuclear reaction using the isotopic symbols:

To remain consistent with your text book author, we will use the Greek symbol for the positron.

We can check that the reaction is balanced by comparing the sums of the lower numbers on each side and the upper numbers. There should be no net change.

Another example of positron decay is found in K-38: