The answers are as follows.
a. 9x
In order to get this, you need to reevaluate 64 as a base of 4. Since 64 is equal to 4^3, we can rewrite the right side as
64^3x = (4^3)^3x = 4^9x
Which gives you the first answer.
b. 10
Similar to the first problem, we need to express 16 as a base of 2. 2^4 is equal to 16, so we use that in it's place and simplify.
16^5/2 = (2^4)^5/2 = 2^20/2 = 2^10
c. Sqrt(7.31)
This one is more simple. Raising something to a 1/2 power (.5) is the same as taking the square root.