Factorise 9 X square + Y square + Z square - 6 X Y + 2 Y Z - 6 x z hence find its value when X is equals to 1 by equal to 2 and Z = 2 - 1

a) Factor 9x² + y² + z² - 6xy + 2yz - 6xz

This looks a lot like

(x+y+z) ² = x² + y² + z² + 2xy + 2yz + 2xz

except instead of x there's (-3x). So this must be

(-3x+y+z) ² = (-3x) ² + y² + z² + 2 ((-3x) y + (-3x) z + yz)

= 9x² + y² + z² - 6xy + 2yz - 6xz

Looks good.

Answer: (-3x+y+z) ²

Since it's a square we could also negate the inside and write this (3x-y-z) ²

b) I'll guess that says we want to evaluate this when x=1, y=2, z = - 1

(-3 (1) + 2 - 1) ² = (-2) ² = 4

Answer: 4