Answer:
x = 10
Step-by-step explanation:
21 = 4x - 9 - x
21 = 3x - 9
30 = 3x
10 = x
Given
21 = 4x - 9 - x, that is
21 = 3x - 9 ( add 9 to both sides )
30 = 3x ( divide both sides by 3 )
