Combining the equations is simple, though it won't give you the answer you wish for:
fflow =
0.0118x^2 - 0.3454x + 343.43 = 0.0012a^2 - 0.9261a + 355.21
or
0.0118x^2 - 0.3454x + 343.43 - 0.0012a^2 + 0.9261a - 355.21
= 0
I think your equations are a bit suspect - as you can see, your expressions mean that you can use weight OR alt and either will give the same answer.
What you seek is an expression in which alt (a) and weight (x)appear together on the same side of the equals sign, e.g.:
Fflow = Ax^n + Ba^n + C then you can solve for Fflow
If you have a graph or tables that plot fuel flow for weight/ alt combinations, or that show fuel flow against another independent variable that can be related to weight or alt or both, that is where you need to start working...
As always - I may have missed something obvious (does happen!) so feel free to pick my theory apart!