Answer:

Step-by-step explanation:
Given


Required
Eliminate a
Multiply the first equation by -4

Add to the second equation

Solve brackets

Open bracket


At this point, a has been eliminated;
From the list of given options, the option that answers the question is 
840=2 times 2 times 2 times 3 times 5 times 7
3 x - 7 = 5 x + 13
3 x - 5x = 13 + 7
-2x = 20
x = 20 / -2
x = - 10