That's the right way to do it, now notice that it is equal to the equation \(\displaystyle x^2 + 9x - 10^k = 0\), which is a quadratic equation in \(\displaystyle x\) with a constant term in \(\displaystyle k\). Therefore you can apply the quadratic formula to isolate \(\displaystyle x\) and solve it in terms of \(\displaystyle k\).