The function
$$f(x) = \left\{ \begin{array}{cc} 5x + a & (x < -2) \\ x^{2} - a & (x \geq -2) \end{array} \right.$$
is continuous on the set of all real numbers. What is the value of the constant $a$? [3 points]\\
(1) 6\\
(2) 7\\
(3) 8\\
(4) 9\\
(5) 10