Apitcher of water is three-fourths full. four cups of water are served from the pitcher. then the pitcher is two-thirds full. how many cups of water can the pitcher hold?

So hmmm let's say, the pitcher has "x" cups when full
so, it was (3/4)x then 4 cups came out, went down to (2/3)x

now, if we subtract (2/3)x from (3/4)x, their difference will be the 4cups that were served

thus   

and surely, you'd already know what "x" is