Answer :
Solve equation [2] for the variable y
// Plug this in for variable y in equation [1]
// Plug this in for variable y in equation [3]
// Solve equation [3] for the variable z
// Plug this in for variable z in equation [1]
// Solve equation [1] for the variable x
// By now we know this much :
x = 1 y = 6x+3z-12 z = -2x+3// Use the x value to solve for z
// Use the x and z values to solve for y
y = 6(1)+3(1)-12 = -3