Hi Goaterino,
Welome to the Web2.0calc forum 
My answer is really just the same as Heureka's and Max's. Thanks Heureka and Max :)
If you still do not understand it would be helpful if you try to explain which bit you do not understand :)
5(sqrt5) can be written in the form 5^k
What is the value of k
Things you must know
\(\sqrt5=5^{1/2}\\ 5=5^1\\ 5^a*5^b=5^{a+b} \)
so
\(5(sqrt5)\\ =5^1*\sqrt5\\ =5^1*5^{1/2}\\ =5^1*5^{0.5}\\ =5^{1+0.5}\\ =5^{1.5}\)
So k=1.5
Heureka has used a comma instead of a decimal point - some countries do that :)