You are being asked to answer whether two sets meets a definition (bijection). Your first set are your x values, these are your inputs to the function. So say the function is f(x)= x^2, the first set is the values of x say (1,2,3). The second set is your ouput. So in this case (1^2, 2^2, 3^2) = (1,4,9). Again the two sets are (1,2,3) and (1,4,9). Now the question asks how do you know if this a bijection. A bijection means that each value x corresponds to a unique value y and each value y corresponds to a unique value x. So 1 -> 1, 2 -> 4, 3->9. So this set is a bijection. Sets that wouldn't be bijections are (1,2,3,5) -> (1,4,9) x value 5 does not correspond to any value y, (1,2,3) -> (1,4,9,16) y value 16 does not correspond to any x value, (1,1,3) -> (4,2,3) 1 value x corresponds to multiple y values, (4,2,3) -> (1,1,3) 1 value corresponds to multiple x values.