I assume this might be
4^t=8^(-t+15) if so, we can write
(2^2)^t = (2^3)^(15 - t)
(2)^(2t) = (2)^[3 *(15 - t)] and since we have like bases, we can solve for the exponents
2t = 3(15 - t)
2t = 45 - 3t add 3t to both sides
5t = 45 divide both sides by 5
t = 9
