3x - 4y = 14
x + y = -7
use the second equation and solve for any variable (I'm going to solve for y)
y = -7 - x
now that u solved for the variable you plug it into the equation u haven't used yet
3x - 4(-7 - x) = 14
distribute
3x+28+4x=14
add the like terms
7x+28=14
subtract 28 from both sides (cuz now u just want to find x)
7x=-14
divide
x=-2
Now find y (you can use the original second equation)
(-2) + y = -7
solve for y
y= -5