Solve for x,y &z of the linear equation

Solve the system of linear equations and check any solutions algebraically. (If there is no solution, enter NO SOLUTION. If the system is dependent, express x, y, and z in terms of the parameter a.)

x	+	y	+	z	=	12
2x	−	y	+	z	=	18
3x			−	z	=	21