Firstly, the image of y=sin(2x+π/3) is made, and the symmetry axis is x=π/6.

It is found that if the equation sin(2x+π/3)-a=0 is in the interval 0, π/2 holds.

That is, the straight line of y=a intersects with two points of the image of y=sin(2x+π/3) in the definition domain.

Therefore, x 1, x2 must be within the definition domain of 0, π/6, and be symmetrical about the symmetry axis.

That is (x 1+x2)/2=π/6.

So x 1+x2=π/3.