I can get the answer using symbolic package in octave but when I go to convert them back to doubles I get an error see code and error below. (Thanks to Mike Miller for getting me this far)
pkg load symbolic
syms a b
assume(a, 'positive')
assume(b, 'positive')
x1=2;y1=12;
x2=8;y2=768;
sol = solve (y1==a*b^x1, y2==a*b^x2, a, b)
a_val=double(a)
b_val=double(b)
I get an error
error: cannot convert to double
error: called from
double at line 126 column 7
test_symbolic at line 12 column 6
I'm use Octave 4.0 Ubuntu 16.04 64bit
symbolic *| 2.2.4 | /usr/share/octave/packages/symbolic-2.2.4 (from Ubuntu repos)