Remark
No matter what kind of triangle it is, the angles must add up to 180, just as you said.
Suppose we say
A = 6x - 9
B = 9x + 1
C = y We need an expression for C or y.
A + B + C = 180
6x - 9 + 9x + 1 + y = 180 Collect the like terms on the left.
6x + 9x + 1 - 9 + y = 180
15x - 8 + y = 180 Add 8 to both sides.
15x + y = 180 + 8
15x + y = 188 Subtract 15x from both sides.
y = 188 - 15x <<<<<<<<< Answer
