I don't get it, help me please.

The nested functions at the end mean that you put 16 into the function g, then take what you get and put it in the t function (sometimes written as (t o g)(x) )

You can rewrite function t as t(x)=3-√x, because the g function inside of it means that you subrtact the g of x from 3

f(g(16))........means that we put 16 into g, first.....and evaluate....

Then...we put that result into f

So

g(16)  =  sqrt (16)  = 4

And

f(4)  = 3 - g(16)  =   3 - sqrt (16)  =  3 - 4  =   -1