What? SQRT(4)=2 and not just -2

I was doing simple linear approximations today and this came up.
Apparently, sqrt(y) where y=4 must be 2 and not also -2, or else the equation is wrong and doesn't work.
It's because there is no change in sign if you square root both sides to find the answer.

answer^2 = 4
answer = sqrt(4)

The sign is naturally positive outside the square root.

The Question:






Find the linear approximation of the function at the point (-7, 4).