Solve for all possible values of x. √21 - x = x - 1

I do not know if I'm doing it right and also I don't how to factor so I'm really having a problem with this and would appreciate it if I could get an answer and explanation.

Twelve less than a number is at most 20

Two numbers have a sum of 93. if one number is p, express the other number in terms of p.

Choose the domain & range of the relation shown in the graph: domain: -1, 0, 1, 2, 3 range: -3, -1, 0, 3 domain: -3, -1, 0, 3 range: -3, -1, 0, 3 domain: -3, -1, 0, 3 range: -1, 0, 1, 2, 3 domain: 3, 1, 0, 3 range: -1, 0, 1, 2, 3