The square of a number is 6 more than 5 times the number. what is the number?

X^2 = 6+5x => x^2 - 5x - 6 = 0 => (x-6)(x+1)=0

This riddle has two solutions:

x=6 or x=-1

indeed 6*6 = 36 = 5*6+6
indeed -1*-1 = 1 = 5*-1 + 6

The -1 is kind of a surprise, isn't it?