To solve this system of equations by elimination, what operation could be used to eliminate the y-variable and find the value of x? 2x ? 4y = 6 ? 3x + 3y = 9 a) add 3 times the second equation to 4 times the first equation b) add 4 times the second equation to 3 times the first equation c) subtract 3 times the second equation from 4 times the first equation d) subtract 4 times the second equation from 3 times the first equation