This is because simplifying the equation before solving it will lead you to the answer at a much faster rate. For example, in the given equation, the long way would be to distribute 2 to (x+3). Then, you transpose them and combine like terms. Until you get to x + 3 = 8. But if you simplify it first, you will see that it is straightforward. It will lead you to the answer right away which is x + 3 = 8. It saves you from the hassle of doing more algebra :)
This is Transitive Property.
Blame me if you get it wrong :\
Answer: 
Step-by-step explanation:
Given
from the figure, we can write
