We can't square root such a number in a standard way because nothing powered by 2 may give us 18.5. However we can find a rounded decimal with a lot of digits on the right hand side of the decimal point. In this case it will be:
√18.5 = 4,3011626335213133858647367675249
We can round it further so it looks much nicer:
√18.5 ≈ 4,3
Note that (4,3)² gives us 18,49, so it's a pretty close rounding.