What is this and how do you do it? I’m confused...

this means alternate interior angle are congruent. to solve this, you would equal them together.
you would write out the equation, and then subtract 6x on both sides.
7x-7 = 6x+11
-6x -6x
this will give you
1x-7 = 11
you then add 7 on both sides
1x - 7 = 11
+ 7 +7
1x = 18
divide by 1 to solve for x
1x÷ 1
18÷ 1
x=18