Va rog răspundeți-mi repede. Dau coroana.

A={x€Z / 21/2x+3 €Z}
A=?
{21}/{2x+3} € Z
=> 2x+3 € D21
D21={-21, -7, -3, -1, 1, 3, 7, 21}
=> 2x+3 € {-21, -7, -3, -1, 1, 3, 7, 21}
=>2x € {-24, -10, -6, -4, -2, 0, 4, 18}
=> x € {-12, -5, -3, -2, 0, 2, 9}
x € Z
=> S={-12, -5, -3, -2, 0, 2, 9}