Which statement is true about the difference 2√7-√28

It seems like there are answer choices but you forgot to post them. If you want to simplify then

2*sqrt (7) - sqrt (28) = 2*sqrt (7) - sqrt (4*7)

2*sqrt (7) - sqrt (28) = 2*sqrt (7) - sqrt (4) * sqrt (7)

2*sqrt (7) - sqrt (28) = 2*sqrt (7) - 2*sqrt (7)

2*sqrt (7) - sqrt (28) = (2-2) * sqrt (7)

2*sqrt (7) - sqrt (28) = (0) * sqrt (7)

2*sqrt (7) - sqrt (28) = 0

So the whole complicated expression simplifies to 0 (zero)

This is because sqrt (28) = 2*sqrt (7) when you simplify sqrt (28)

Side Note: "sqrt" is shorthand for "square root"