To calculate the temperature $$T$$ of the Sun from Earth, one must first find out what light the Sun emits. The Sun emits a polychromatic light, that is, infrared light, visible light, and even X-rays and gamma rays. You need to figure out the wavelengths $$\lambda$$ of each of these radiations and with what intensity ("how bright"?) they are emitted. If you plot all wavelengths with their intensities in a diagram, you get a so-called spectral intensity distribution of the sun. From this distribution you have to read off the wavelength $$\class{blue}{\lambda_{\text{max}}}$$ which has the highest intensity (maximum of the function).