After losing 3 baseball cards, Peter gave half of his remaining cards to Bobby. If Peter gave Bobby 9 baseball cards, how many cards did Peter start with? Linear equation:
1
2 (x - 3) = 9

Bobby claims that Peter started with 21 cards. Which statements are true about verifying Bobby’s claim? Check all that apply.

Substitute 21 for x in the original equation.
Substitute any number for x in the original equation.
Bobby’s claim that x = 21 is correct.
The result of verifying Bobby’s work is 9 = 9.
The result of verifying Bobby’s work is 21 = 21.