First, add 84 to both sides, to isolate the exponential function.
Then, take the logarithm of both sides:
(2x+3)*log 5 = log 84
Solving for 2x+3, we get log 84 - log 5, and so
2x = -3 + log 84 - log 5. Dividing both sides by -3, we obtain the final answer:
-3 + log 84 - log 5
x = -----------------------------
2