If the point of intersection of the lines (x+1)/3=(y+a)/5=(z+b+1)/7 and (x-2)/1=(y-b)/4=(z-2 a)/7 lies on x y -plane, then the value of a + b is:

If the point of intersection of the lines $\displaystyle \frac{x+1}{3}=\frac{y+a}{5}=\frac{z+b+1}{7}$ and $\displaystyle \frac{x-2}{1}=\frac{y-b}{4}=\frac{z-2 a}{7}$ lies on $\displaystyle x y$-plane, then the value of $\displaystyle \mathrm{a}+\mathrm{b}$ is :