Explanation

$$abc=?\text{and }ac=2$$

You want to find the value of $b$ in terms of $a$, assuming:.

$$abc\text{ is implied to be known, or }abc=someexpression.$$

If we assume:

$$abc = a cdot b cdot c quad text{and} quad ac = 2,$$

Then:

$$abc=a\cdot b\cdot c=b(ac)=b\cdot 2=2b$$

So:

$$$$
abc = 2b Rightarrow b = frac{abc}{2}