The sumof the angle measures of any triangle is 180 degrees. suppose that one angle in a triangle has a degree for 6x-9 and another has a degree measures 9x+1. write an expression for the degree measure of the third angle in the triangle.

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