Because f (x+t) >; =2f(x), replace them with f(x+t)=(x+t)*2, 2f(x)=2x*2, and you can get (x-t) * 2.

To put it simply, f(x+t)≥2f(x), but f (x+t) is 2f (x), so f (x+t) = 2f (x) = 2x * 2 = (√ 2x) * 2 = f (√ 2x).