The function \( f(x) = 1,500x + 300 \) represents Manuel's total earnings after \( x \) months, where \( 1,500x \) represents his monthly salary for \( x \) months and \( 300 \) represents the hiring bonus.
To find out how much he received as a hiring bonus, we look at the constant term in the function, which is the amount that does not depend on \( x \).
Therefore, Manuel received a hiring bonus of $300.