Answer:
x = 20
Explanation:
(x+21)+(2x+9)=90
x + 21 + 2x + 9 = 90
combine numbers on left side:
x + 30 + 2x = 90
combine x terms on left side:
3x + 30 = 90
subtract 30 from both sides:
3x + 30 -30 = 90 - 30
3x = 60
divide both sides by 3:
3x/3 = 60/3
x = 20