You are going to need to raise it to a negative exponent.
4^c = 1/16
Rewrite 1/16 as a exponential with a base of 4: 1/(4^2).
Remember that when you move exponentials from the numerator to the denominator (or vice versa), the exponent changes signs.
In other words, a positive exponent becomes a negative exponent. The opposite is true if you have a negative exponent.
4^c = 1/(4^2)
4^c = 4^-2
When you two exponentials equal to each other and their bases are the same (in this case, both of their bases are 4), then you know that the exponents are equal to each other.
c=-2
