Question

what temperature can a solar flare reach

Answer 1

I'm sure you can find this information by searching online. If you wish to actually be specific in some of your calculations though:

Go to this site: http://en.wikipedia.org/wiki/Solar_flare

There is a section on "Classification" of solar flares. You can look at the flux's that are given for different types.

To relate flux to temperature, there is a handy stefan-boltzmann constant.
\[F = \sigma T^{4}\]So you can solve for T as follows:\[T = (\frac{F}{\sigma})^{1/4}\]the constant is equal to: 5.67 x 10^-8

Flux F is measured in W/m^2
sigma is in W/m^2K^4

You'll get temperature in Kelvin K