What's the answer for this problem because i don't understand

Use the distance formula:
d((x1,y1),(x2,y2))
=sqrt((x2-x1)^2+(y2-y1)^2)
=sqrt((5-0)^2+(6-0)^2)
=sqrt(25+36)
=sqrt(61)
Preceding line gives the exact answer.  Calculators will give only an approximation.