Question

F6P = 0.504(1.3 − F6P)
-solve for F6P?

Answer 1

Don't let the F6P throw you. Just call it x.
x = 0.504(1.3-x)
x = 0.6552 - 0.504x
x+0.504x=0.6552
1.504x = 0.6552
x = 0.6552/1.504
x = 0.43564 which rounds to 0.44 based on the two significant figures in 1.3. Check my work.

Answer 2

F6P = 0.436