Answer:
x=8
Step-by-step explanation:
first you need to find the unknown corner so you do 180-71= 109
then find the interior angle degree (n-2)*180 (n=number of sides) so (4-2)*180=2(180)=360
then set everything equal to 360:
10x+6+13x-2+8x-1+109=360
(combine like terms)
31x+112=360
(subtract 112 form both sides)
31x=248
(divide both sides by 31)
x=8
