a==aa- 2 aab
ad== aa (mm^(.5))- 2  aab mm

Clear[a,ad,aab,aa,mm]

Solve[{a==aa- 2 aab,ad== aa (mm^(.5))- 2  aab mm},{aa, aab}]

mm=1.62361/3.15946

a=2884.8

ad=2091

Solve[{a==aa- 2 aab,ad== aa (mm^(.5))- 2  aab mm},{aa, aab}]