<-- Take the fourth root of both sides
What number is this? Well if you recall that ...
Of course, this is the principle value of the solution. You might note that x = -8 also solves the equation. (The easiest way to see this is to take and take the square root of both sides twice.) There are also two complex solutions. I can give you the general solution if you like; it's not much harder than what I derived here.