Answer
$$z=.339, -1.179$$
Work Step by Step
There are many ways to solve these problems. However, it is beneficial to use the quadratic formula, for this will work for almost any expression. Hence, we find:
$$ 100z^2+84z-40=0\\ z_{1,\:2}=\frac{-84\pm \sqrt{84^2-4\cdot \:100\left(-40\right)}}{2\cdot \:100}\\ z=\frac{-21+\sqrt{1441}}{50},\:z=-\frac{21+\sqrt{1441}}{50} \\ z=.339, -1.179$$