First, we must expand and simplify :
\(x + y+z = 9\)
\((x + y+z)^{2} = (9)^{2}\)
\(x^{2} + y^{2}+z^{2} + 2(xy + yz+zx) = 81\)
And Than, Enter the value :
\(x^{2} + y^{2}+z^{2} + 2(26) = 81\)
\(x^{2} + y^{2}+z^{2} + 52 = 81\)
\(x^{2} + y^{2}+z^{2} = 81- 52\)
\(\boxed{x^{2} + y^{2}+z^{2} = 29}\)
