Use Lagrange multipliers. The Lagrangian is
[tex]L(x,y,z,\lambda)=5x+4y+z+\lambda(x^2+y^2+z^2-1)[/tex]
with partial derivatives (set equal to 0)
[tex]L_x=5+2\lambda x=0\implies x=-\dfrac5{2\lambda}[/tex]
[tex]L_y=4+2\lambda y=0\implies y=-\dfrac4{2\lambda}[/tex]
[tex]L_z=1+2\lambda z=0\implies z=-\dfrac1{2\lambda}[/tex]
[tex]L_\lambda=x^2+y^2+z^2-1=0[/tex]
We can substitute the first three equations into the fourth to solve for [tex]\lambda[/tex]:
[tex]\dfrac{25}{4\lambda^2}+\dfrac{16}{4\lambda^2}+\dfrac1{4\lambda^2}=1[/tex]
[tex]\implies 42=4\lambda^2\implies\lambda=\pm\sqrt{\dfrac{21}2}[/tex]
Now use these values of [tex]\lambda[/tex] to solve for [tex]x,y,z[/tex], which should give you two critical points at [tex]\pm\left(\dfrac5{\sqrt{42}},\dfrac4{\sqrt{42}},\dfrac1{\sqrt{42}}\right)[/tex], which would respectively give a maximum and minimum value of [tex]\pm\sqrt{42}[/tex].
