Explanation:
you heard of the Pythagoras formula for rectangular triangles ?
c² = a² + b²
c is the Hypotenuse (the side opposite of the 90 degree angle), a and b are the other 2 sides touching the 90 degree angle.
so, in your case here we have therefore
x² = 8² + 6² = 64 + 36 = 100
x = sqrt(100) = 10
you know "sqrt" is an international abbreviation for square root : the opposite of squaring a number.