Maths-matrix

x-2y+z=5Consider the systemn of linear equations(*)｛x-y-z=4 , where a is a real number.x+y+az=2a) Find the range of values of a such that(*) ha s a unique solution. Solve (*) when it has a unique solution.(I find that a≠5；x=3, y=-1, z=0) b) Suppose that a=-5. Solve (*)(I solve that x=t, y=(2t-9)/3, z=(t-3)/3x-2y+z=5c) Supper that (x,y,z)satisfies｛x-y-z=4 . Find the least values of (x^2)+(9y^2)+3z and x+y+az=2the corresponding valuesof x,y,z. a, b part 我已經計了,麻煩幫忙計c part