Answer:
x = 1/3 (arccos (-7/15) + 2πn)
Step-by-step explanation:
For solving for x in

we first divide both sides by 3 to get


taking the arccos of both sides gives


finally, dividing both sides by 3 gives

where n is an integer.
We can find the numerical answer for x by using a calculator to find the value of arccos(-7/15).