If the length of the sub normal is equal to length of sub tangent  at a point (3,4) on the curve y=f(x) and the tangent at (3,4) to y =f(x) meets the co-ordinate axes at A and B,then maximum area of the$\mathrm{△}\mathrm{OAB}$ where ‘O’ is origin is $\frac{\mathrm{a}}{\mathrm{b}}$then a-b= _______