express in log10 form

16 ^(x-4) = 9 ^(-9x)
log ₁₀ 16 ^(x-4) = log ₁₀9 ^(-9x)
(x-4) log ₁₀ 16=-9x log ₁₀ 9
x(log ₁₀ 16 + 9 log ₁₀ 9)=4 log ₁₀ 16
xlog ₁₀ (16*9 ⁹)=4 log ₁₀ 16
x=4 log ₁₀ 16/log ₁₀ (16*9 ⁹)