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"