Which value of x satisfies both -9x + 4y = 8 and -3x − y = 4 given the same value of y?
a.-1/7
b.-7/9
c.-8/7
d.-9
e.-4/3

Use simultaneous equations
put y as the subject so that 4y = 8 + 9x
and y the subject for the other equation so that y = -3x - 4
multiply this equation by 4 so that the y's equal each other 
your new equations are 
4y = 8 + 9x and 4y = -12x - 16
you can then say 8 + 9x = -12x - 16
then 21x = -24
x = -24/21 = -8/7 so C