Hi Motaz. Let me demonstrate with a similar question:

Let z = log₅ (1/625).

Start by realizing that if the log is in base 5, we want to exponentiate with base 5 on both sides:

5^z = 5 ^{log₅ (1/625)}

On the right hand side, the exponentiation and the log cancel each other, because they have the same base of 5. So this simplifies to

5^z = 1/625

From here, it is just normal work with exponents:

5^z = 625^-1
5^z = (25²)^-1
5^z = [(5²)²]^-1
5^z = 5^-4
z = -4.

Now you can do your question exactly the same way.

Cheers,
Stephen La Rocque.