Using the numbers [12, 21, 39, 54], create an equation that equals 99. You can use basic arithmetic operations (+, -, *, /) and each number can only be used once. Show your work in <think> </think> tags. And return the final equation and answer in <answer> </answer> tags, for example <answer> (54 - 39) * 2 + 12 = 99 </answer>. Please note, keep in mind that division here results in an integer result.