If the image of the point \( P(a,2,a) \) in the line \( \dfrac{x}{2}=\dfrac{y+a}{1}=\dfrac{z}{1} \) is \( Q \) and the image of \( Q \) in the line \( \dfrac{x-2b}{2}=\dfrac{y-a}{1}=\dfrac{z+2b}{-5} \) is \( P \), then \( a+b \) is equal to \( \_\_\_\_\_\_\_\_ \).
