Since its a triangle we know that all 3 interior angles added up is gonna eqaul 180 degrees. So we already have 2 angles which are 21 and 17. You add those up and you’d get 38. Now since the final angles is unknown we can just make a simple equation of 38+x=180 since we know 2 angles and we need 1 more and a triangle is 180 degrees alli ddded up together. You’d get 142 for that angle. X is adjacent to that angle and they form a straight line so it also has to be 180. We got 1 side and so we subtract 180-142= 38. X will be 38.
For (2)
Since its 2 parallel lines cut by a transversal and we have two same side interior angles we know its gonna be supplementary Which means that both those angles added up will be 180. Now we have the two different angles so we add them up and solve for x. (10x+17)+(3x+7)= 180. Commbine like terms which will turn out to be 13x+24=180. Subtract 24 on both sides. 13x= 156. Solve for x by diving 13 on both sides which will lead to x= 12
