I see we're really working on Calculus today, Dalia! :)
Alright, let's tackle this one. First, a rewrite...
y = 4 x^{1/2} (Derivation doesn't like roots; it likes exponents.)
Now the derivative...
dy = 2 x^{-1/2} dx
Solving the differential at the point given...
dy = 2 (2)^{-1/2}(0.4)
= (2*0.4)/√2
= (0.8)/√2 OR 4 / (5√2) OR .56569.
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