This is a simple application of english system conversions. I'm afraid that's all there is to it, you're just going to have to commit these to memory.
That said, I shall walk you through the problem. This one is fairly easy.
We know that in one pint there are two cups. As such, in two pints there are four cups, and in three pints there are six cups.
Therefore, there are 6 cups in 3 pints.
Hope this helps :)