x4-y4=(x2-y2)(x2+y2)=(x-y)(x+y)(x2+y2).
So if x4=9b4, then x2=3b2 and x=b√3; y4=200=2×100, y2=10√2, y=√10 × 4√2 or 4√200
9b4-200=(b√3-4√200)(b√3+4√200)(3b2+10√2).
These are the real factors but they are irrational. √3=1.7321, √2=1.4142, √10=3.1623, 4√200=3.7606 (all approximate).