Answer:
k=6
Explanation:
a line is 180deg and since there is a 90deg angle there, you can subtract it from 180, leaving you with 90.
(7k + 8) + 40 = 90 >> subtract 40 from both sides
7k + 8 = 50 >> subtract 8 from both sides
7k = 42 >> divide both sides by 7 to get k alone
k = 6