Answer:
10.5
Step-by-step explanation:
3x - 9 = x + 12
Subtract x from both sides
2x - 9 = 12
ad 9 to both sides
2x = 21
Divide both sides by 2
x = 10.5
[tex]these \: sides \: are \: equal \: (rhombus) \\ x + 12 = 3x - 9 \\ 2x = 21 \\ x = 10.5[/tex]
