The general equation is that for every red tile, there must be 12 blue ones.
Algebraically this is expressed as b = 12r where r is red and b is blue. Your teacher might like you to express it as y = 12x where y is blue and x is red. The meaning is the same.
For beginners it might be easier to say b = 12r. So if you have 15 reds then r = 15
b = 12 * r
b = 12 * 15
b = 180 blue tiles.
Sometimes it helps to draw it out...
(17)
C_______M_________Q
|___________________|
21
therefore, CM + MQ = CQ
17 + MQ = 21
MQ = 21 - 17
MQ = 4 <===
BbbbbbbbbbbbbbbsbsbsbabHsnabHbnasbsbsbsbnznznznznz
Answer:
The step 4 is not what he should have done. But the wrong is step 5, because if he added -2x both sides, it would give the following equation:

and not 
Note that in step 4 he should have added 2x and not -2x to both sides, but we still have the equality. The problem is that step 5 is totally wrong.
I don't know what answer is expected to this question, but I would mark step 5, because this is mathematically wrong.
Solving the equation

Using distribution

Add
both sides


Add 12 both sides

Divide both sides by 5

Answer:
Answer given below
Step-by-step explanation:
D = - 6 x 5 = - 30