I presume you need to curve fit (0,0),(12,20),(30,30) such as,Originally Posted byasian5

Subsituting this into this equation yields,

---------

From equation 1 we have,

Substitute that into equation 2 and 3 thus,

Thus,

Divide these equation thus,

Cross multiply,

Now raise to that power,

Thus,

Thus,

Open sesame,

Thus,

Thus,

<to be countinued>

this gives you b.