Answer:
9
Step-by-step explanation:
All triangles (3 sided polygons) have angles added up to 180, so we can do the following:
(7x-1) + (7x+8) + (5x+2) = 180° <em>- equation</em>
7x-1 + 7x+8 + 5x+2 = 180° <em>- "distribute"</em>
19x - 1 + 8 + 2 = 180°<em> - combine the x terms</em>
19x + 9 = 180° <em>- combine the integer terms</em>
19x = 171 <em>- subtract 9 form both sides</em>
x = 9 <em>- divide both sides by 19</em>