kx + z = 3 means z = 3 - kx. Substitute this into the first two equations:
x + y + 3 (3 - kx) = 10 ==> (1 - 3k) x + y = 1
-4x + 3y + 5 (3 - kx) = 7 ==> (-4 - 5k) x + 3y = -8
Multiply through the first equation by -3 :
-3 ((1 - 3k) x + y) = -3 (1) ==> (-3 + 9k) x - 3y = -3
Add this to the second equation to eliminate y :
((-3 + 9k) x - 3y) + ((-4 - 5k) x + 3y) = -3 + (-8)
(-7 + 4k) x = -11
Normally, you would solve for x by dividing both sides by -7 + 4k. But you can't do that if this turns out to be equal to 0, which happens for
-7 + 4k = 0 ==> k = 7/4
