I can’t figure out how to answer this question for the life of me. Please help!

This is from the Big Book. Test 7, Section 3, Question 9

I can’t figure out how to answer this question for the life of me. Please help!

This is from the Big Book. Test 7, Section 3, Question 9

the answer is D isn’t?

because using triangle similarity we can get 8/6 = x/(6+a) (a is the line along 6)

but that means x can be any number as long as these both are in the same ratio as 8/6.

So, unless we know the value of 6+a, we can’t find out the value of x. Gotcha.

Thanks!