3z - 4 = 6z - 17. First try to take all the zs to one side of the equation
preferrably to the left. And the sign of any expression
changes on crossing the equality sign.
3z-6z = -17+4
-3z = +4-17
-3z = - 13. Divide both sides by -3.
-3z/-3 = -13/-3
z = 13/3.