The 2 small black lines through the sides of the triangle indicate that the 2 sides are the same length.
Because the 2 sides are the same the 2 bottom angles are also the same.
This means that 9x +3 = 84
Now we can solve for x:
9x +3 = 84
Subtract 3 from each side:
9x = 91
Divide both sides by 9:
x = 81 / 9
x = 9
