I assume this is :

3^{x + 3} = 7776 and we can write

3^3 * 3^{x} = 7776 divide both sides by 3^3 = 27

3^{x} = 288 take the log of both sides

log 3^{x } = log (288) and we can write

x * log 3 = log (288) divide both sides by log 3

x = log (288) / log (3) ≈ 5.155