Question 26356
Multiply first equation by -2.  We get -4x - 6y = -42.


Now, add the altered first equation to the second equation.  We get -4y = -20.


Solve -4y = -20.  We get y = -20/-4 = 5.


Now that we have the value for y we can go back and substitute this value in either of the original equations and solve for x.  Let's use the first equation.
2x +3y = 21
2x +3(5) = 21
2x +15 = 21
2x = 21 - 15 = 6
x = 6/2 = 3


This method for finding two unknowns with two equations is called the addition/substitution method.  There are other methods for doing this problem also.