There are special properties to certain right triangles.
A 3-4-5 triangle will always be a right triangle. 9 divides into 3, and 15 divides into 5, so you can use ratios to find b.
Since 9 divides into 3, we will use this to set up proportions for the missing value:

Cross multiply the fractions.

Divide both sides by 3.

The missing value is
12.
it's really just your own opinion of which is difficult or easier but in my opinion Substitution is more difficult because you have to substitute in the different equations, and it can take more time. I personally like Elimination best because it cancels out variables and can be quicker to use. hope this helps:)
Answer:
Should be 4/7
Hope it helps have a good day!