HI

I guess there is a typo . See red.

This is just a very basic application of the sine rule and it's very obvious . Just draw a triangle and you should be able to see.

8/ sin 40 = 6/sin B

Then the sum of angles of a triangle will tell you angle C .

so now use the sine rule once again .

8/sin 40 = c/ sin C

OR you can use the cosine rule , its faster .

8^2=6^2+c^2-2(6)(c)cos 40 .

solve for c .