Answer
14 mm.
Work Step by Step
$$d sin\theta=m\lambda$$
For small angles, the tangent and sine are approximately equal.
The tangent is the spacing from the centerline, x, divided by the distance from the slits to the screen, L.
$$d\frac{x}{L}=m \lambda$$
$$x=\frac{m \lambda L}{d}$$
Let the 480 nm light be subscript 1, and the 650 nm light be subscript 2.
In the 2 cases, the slit spacing d and the distance from the slits to the screen, L, stay the same.
$$x_1=\frac{m_1\lambda_1 L}{d}$$
$$x_2=\frac{m_2 \lambda_2 L}{d}$$
Solve for $x_2$.
$$x_2=\frac{\lambda_2 m_2}{\lambda_1 m_1}x_1=\frac{(650nm)(2)}{(480 nm)(3)}16mm\approx 14 mm$$