I won't write about the specific process, but I'll tell you why we get it by eliminating C, and the range of B is, and the range of C is.

First of all, when x2, b and c all take the interval boundary, we must get-16.

However, if x2= 1, then if the boundary value is to be taken, b must only be equal to 0.

If x2=2, then if you want to take the boundary value, b must only be equal to-1.

but

So, if you leave b as a variable and you find-16 when x2=2 b =0, then-16 will not appear at all.

But I checked again, and even if I keep B, there will be no problem. Or-10.