Trigonometry (10th Edition)

Since $7$ is a natural number and natural numbers are a subset of the set of real numbers, $7$ is a real number. Also, for a complex number $a + bi$, if $b = 0$, then $a + bi = a$, which is a real number. Thus, the set of real numbers is a subset of the set of complex numbers. Therefore, 7 can be identified as a complex number as well.