Answer:
   C.  postulate; theorem
Step-by-step explanation:
While a "given" is taken as true without proof, and a "problem" sometimes involves an assertion that can be proven, the answer choices involving these terms are not correct here. 
The dictionary definitions of "postulate" and "theorem" apply. It is useful to learn the meaning of these terms so you can understand problems and descriptions that use them.
 
        
             
        
        
        
Start with the second expression.  That factors easily into (8x-3)(8x+3).
Since 64x^2 + 48x + 9 has no negative signs, I'd bet that the common factor is (8x+3).  Divide  <span>64x^2 + 48x + 9  by that to verify.
</span>
        
             
        
        
        
1. Subtract 6x from both sides so now you have -2y = -6x + 18
2. Divide everything by -2, then you would have y = 3x - 9
3. The equation is now in slope intercept form so the answer is 3.