Can't get my head round 'sets' in game theory. I am trying to tackle:

Player 1 chooses strategy x1 from [-1,1] and Player 2 chooses strategy x2 from [-1,1]. Player 1 utility function is x1^2/2 + x1x2. Player 2 utility function is x2^2/2 -x1x2. Find the best response function of each player (to pure strategies). Is there a nash equilibrium?

I really have how to do this. Please help.