2x(x - 3) + 7(x - 3)
Can I have an explanation on how it works factors

Step-by-step explanation:

2x(x - 3) + 7(x - 3)

2x^2 - 6x + 7x - 21

reduce

2x^2 + x - 21

if this is equal to 0 then:

2x^2 + x - 21 = 0

factored:

(2x + 7)(x - 3) = 0

How that was done:

Divide the (2x^2) into (2x) and (x).

Divide the (-21) into (+7) and (-3). You choose these numbers so that their sum, when multiplied by (2x) and (-3), adds up to (+x).  

c is the answer

step-by-step explanation:

you didn't post any answer choices, so its not 100% clear what your teacher is looking for

however, log(15/7) is equivalent to log(15) - log(7)

i'm using the rule log(x/y) = log(x) - log(y)