help with some relevant maths for perf calcs

[compressor stall]

I am playing around within excel to create (initially) a simple program that will approximate fuel flows (TPE 331-12JR) at different weights and altitudes. (will live in the pda).

My maths is a little rusty as the following will show.

I have established the following relationships that calculate the FM data to within 1 lb/hr. All at same temp (ISA) and same pwr setting.

let weight(kgs)/100 =x

then: fflow(wt) = 0.0118x^2 - 0.3454x + 343.43



let alt (fl)=a

then: fflow(alt)= 0.0012a^2 - 0.9261a + 355.21

Now...how do I combine them so inputting weight and alt I get the appropriate fuel flow?

(or anyone got a better way to do it!)

thanks (and undoubtedly more qu's to follow!).

(edited to make clearer!)