Do you have any equations that are easy enough for 9th graders but hard enough so that it takes them some time?

I'm substituting for this sick teacher at a school that I had never gone to. I was asked by the headmaster to bring my own material, and it's the first time that I have to substitute for a math teacher. Does anyone have any puzzling equations for ninth graders? I don't want to have to give them a big explanation and I want the equations to be fun so they are on them the whole class (45 min.)