First, we need to find the angle in red that I show in the image below, the angle red form a supplementary angle with the angle of 155° that means if we sum these angles the result must be 180°
We can have the next equation
angle red +155=180
angle red=180-155
angle red=25°
then we need to remember that the sum of the interior angles of a triangle must be 180, so we have the next equation
80+25+x=180
then we isolate the x
x=180-80-25
x=75°
