The goal to proving identities is to transform one side into the other. We can only pick one side to transform while the other side stays the same the entire time. The general rule of thumb is to transform the more complicated side (though there may be exceptions to this guideline).
So I'll take the left hand side and try to turn it into 
One way we can do that is through the following steps:

Since we've shown that the left hand side transforms into the right hand side, this verifies the equation is an identity.
Answer:
-5,3
Step-by-step explanation:
-5x3=-15
-5+3=-2
Answer:
B
Step-by-step explanation:
We can see in the expression that there are 2 x² blocks, 4 -x blocks, and 3 -1 blocks. Therefore, we can write this as
2 * x² + 4 * (-x) + 3 * (-1) = 2x²-4x-3
Comparing this with each answer, we have
A: x²-2x-4 + x² + 2x-1 = 2x²-5. This is not correct
B: 3x²-7x+1-(x²-3x+4) = 3x²-7x+1 -x²+3x-4 = 2x²-4x-3. This seems correct but we can check the other answers to be sure
C: (2x+1)(x-3) = 2x²-6x+x-3 = 2x²-5x-3. This is incorrect
D:
. This is incorrect
3 is the answer they are after 40