x + 7 + 8 + 9 = 345
x + 7 + 8 = 345 - 9
x + 7 +8 = 336
x + 7 = 336 - 8
x + 7 = 328
x = 328 - 7
x = 321
x + 7 + 8 + 9 = 345
x + 24 = 345
x = 345 - 24
x = 321
x + 7 + 8 + 9 = 345
x + 24 = 345
x = 345 - 24
x = 321
x+7+8+9=345
x+24=345
x =345-24
x =321
x+7+9+8=345
x+24=345
x=345-24
x=321
Vậy x=321