Question 1016896
Subtracting the 2nd equation from the 1st equation, and solving for x, we get

{{{x = (z+1)/4}}}.

Now 3 times the 1st equation plus the 2nd equation, and solving for y, we get

{{{y = (13-7z)/12}}}.

Substituting these last two equations into 3x - 3y + 4z, and simplifying, we get
{{{(13z - 5)/2}}}.

Since {{{z> =0}}}, the minimum value of 3x - 3y + 4z is -5/2, while it has no largest value.