Answer:
15=a
Step-by-step explanation:
how I got this is because a triangle is equal to 180 so you have to add all of the things together then you get 12a=180 the reason why I got this is because if you add it so 6x+3a-1+3a+1 so you add common terms then you will equal to 12a then you divide it on both sides so then you equal to 15
