The 'o' notation means function composition. In this case
(g o h)(x) = g(h(x))
The inner function h(x) goes first. Replace x with 25 and simplify to get
h(x) = sqrt(x)
h(25) = sqrt(25)
h(25) = 5
Therefore,
(g o h)(25) = g(h(25)) = g(5)
Now we compute g(5)
g(x) = x-3
g(5) = 5-3
g(5) = 2