x=13
1) Considering that (6x -3), 90 and (9x -12) we can write since the sum of the interior angles of any triangle is equal to 180º
6x -3 +90 +9x -12= 180 Combine Like terms
15x -15 = 180 Add 15 to both sides
15x = 180 +15
15x = 195 Divide both sides by 15
x=13
2) So x = 13 to satisfy the triangle anglular condition, i.e. the sum of their angles is 180º
