This question is called "create equations to solve for missing angles"
its very confusing.

A  

But it needs explanation.

Step-by-step explanation:

The first one hides the fact, but it is indeed the answer.

6x + 12x = 90 degrees is the way the equation should actually be set up.

A does that by adding 90 to both sides to start with.

6x + 12x + 90 = 90 + 90

The reason it does this is to show that the three angles (6x   12x and 90 make up the entire straight angle shown in the diagram.

The answer is going to be the same.

18x = 90                      Combine like terms on the left

18x /18 = 90 / 18          Divide by 18

x = 5

A

Step-by-step explanation:

The 3 given angles form a straight angle and sum to 180°, hence

6x + 12x + 90 = 180 ← is the required equation

where r the answers

step-by-step explanation:

1.3

step-by-step explanation:

3x2=6   9x+6

bring down the 6 on both sides

6+6=12

12= 9x

divide by 9

12/9 =

1.3 =x