Answer:
The number is 2
Step-by-step explanation:
Twice a number (z) = 2*z = 2z
Twice a number (z) added to 21 = 2z + 21
9 times the number added to 7 = 9z + 7
9z + 7 = 2z + 21
Subtract 2z form both sides
9z - 2z + 7 = 21
7z + 7 = 21
Subtract 7 from both sides
7z = 21 - 7
7z = 14
Divide both sides by 7
z = 14/7
z = 2
