which has higher energy a photon of red light with wavelenght 7500 A or a photon of green light with wavelenght 5250 A?

Wavelength of  red light $\left(\lambda \right)$ = 7500 $\stackrel{\circ }{\mathrm{A}}$ = $7500\times {10}^{-10} \mathrm{m}$

As we know :
Energy of light = $\frac{\mathrm{hc}}{\mathrm{\lambda }}$
$$\begin{gathered} \mathrm{where} \mathrm{h} (\mathrm{planck}\text{'}\mathrm{s} \mathrm{constant}) = 6.626\times {10}^{-34}\mathrm{Js} \\ \mathrm{and} \mathrm{c} (\mathrm{speed} \mathrm{of} \mathrm{light}) = 3\times {10}^8\mathrm{m}/\mathrm{s} \end{gathered}$$

$$\begin{gathered} \therefore \mathrm{Energy} = \frac{6.626\times {10}^{-34}\mathrm{Js} \times 3\times {10}^8\mathrm{m}/\mathrm{s}}{7500\times {10}^{-10}\mathrm{m}} \\ = \mathbf{2}\mathbf{.}\mathbf{6504}\mathbf{ }\mathbf{\times }\mathbf{ }{\mathbf{10}}^{\mathbf{-}\mathbf{19}}\mathbf{J} \end{gathered}$$

and for green light wavelength $\left(\lambda \right)$ = 5250 $\stackrel{\circ }{\mathrm{A}}$ = $5250\times {10}^{-10} \mathrm{m}$
 

$$\begin{gathered} \therefore \mathrm{Energy} = \frac{6.626\times {10}^{-34}\mathrm{Js} \times 3\times {10}^8\mathrm{m}/\mathrm{s}}{5250\times {10}^{-10}\mathrm{m}} \\ = \mathbf{3}\mathbf{.}\mathbf{7863}\mathbf{ }\mathbf{\times }\mathbf{ }{\mathbf{10}}^{\mathbf{-}\mathbf{19}}\mathbf{J} \end{gathered}$$

Thus red light  with greater wavelength has lower energy than green light.