Just define stuff, then you can talk about it and write equations about it.
H = Length of Head
B = Length of Body
H + B = Total Length
Now just translate:
"The length of a fish equals the length of its head plus a quarter length of its body."
H + B = H + B/4
You should see immediately that this makes no sense. Let's keep translating, anyway, just for the practice.
"Its body length is 3/4 of its total length."
B = (3/4)*(H + B)
"Its head is 4 inches long."
H = 4 in
Our only problem is that first phrase. Maybe it was just kidding. You should be able to solve if you ignore it and use just the last two clues.
How sure are you taht you copied the problem correctly? It happens.