Let the energy of a photon equal $ E $ and the wavelength equal $ \lambda $. Since the wavelength is inversely proportional to the energy, the product $ E\lambda $ must be equal to some constant, say $ k $. Given photons of red light with a wavelength of $ 7\times10^{-7} $, we can write: \begin{align*} E(7\times10^{-7})&=k\\ \Rightarrow\qquad 7\times10^{-7}&=\frac{k}{E} \end{align*} Now, we are asked to find the wavelength of a photon with 2000 times the energy of red light. Substitute $ 2000E $ for $ E $ in the original expression: \begin{align*} (2000E)\lambda&=k\\ \Rightarrow\qquad \lambda&=\frac{k}{2000E}\\ &=\frac{1}{2000}\cdot\frac{k}{E}\\ &=\frac{1}{2\times10^3}\cdot7\times10^{-7}\\ &={3.5\times10^{-10} \text{ meters}} \end{align*} Therefore, we have $ a+b = \boxed{-6.5} $.