<<a+b=11
<<ba=2ab-7
ba=(10*b)+a
ab=(10*a)+b
a=11-b ; (10*b)+a=2*(10a+b)-7
10b+a=20a+2b+7
10b+11-b=20(11-b)+2b-7
9b=220-18b-7
27b=216
b=8
a=3
ab=38; ba=83
