The sum of a number and its square is 42. which equation can be used to find the two numbers for which this is true? a. x2 + x = 42b. x2 + 2x = 42c. x2 + x + 42 = 0d. x2 + 2x + 42 = 0

If the number is represented by x, then its square is x². The "sum of a number and its square" will be x + x². "Is 42" means this sum is equal to 42, so we have
  x + x² = 42

Of course, the addition is commutative, so we can rearrange the order to
  a. x² + x = 42