Joint Variation and Combined Variation - Definitions

A farmer is building a rectangular pen for his horde of giraffes (there was a deal on Amazon). The number of giraffes he can keep in the pen varies jointly with the area of the pen. $\\$ When he builds a pen that's $20$ m by $20$ m, he can keep $3$ giraffes. But he has $84$ and can only build up to a width of $64$ meters. What does the length of the pen need to be?