Hi, I submitted answers for my homework but got it wrong and I would like to know where I stumbled?

y = 3sqrt(x)

Find delta y when x = 5 and delta x = .2

for this I just figured that delta y would equal: f(5.2) - f(5) = .133

Find the differential dy when x = 5 and and dx = .2

I used the dy = fprime(x) dx formula to get .134

I got wrong results for both. It surprised me but is there something I haven't looked at? Thanks for clarification