$$\ln\left(7^{-x-8}\right)=\ln\left(8^{2x}\right)$$
Expand ln(7-x-8) by moving -x-8 outside the logarithm.$$\left(-x-8\right)\ln\left(7\right)=\ln\left(8^{2x}\right)$$
Expand ln(82x) by moving 2x outside the logarithm.$$\left(-x-8\right)\ln\left(7\right)=2x\ln\left(8\right)$$
Solve the equation for x.$$x=-\frac{8\ln\left(7\right)}{\ln\left(7\right)+2\ln\left(8\right)}$$