find the set of numbers each of which is 30 less than its own squares.
To me that says:
x^2 - x = 30
x^2 - x - 30 = 0
(x-6)(x+5) = 0
x = 6 or x = -5
your set is {-5, 6}
x^2 - x = 30
x^2 - x - 30 = 0
(x-6)(x+5) = 0
x = 6 or x = -5
your set is {-5, 6}