To solve this equation, we can use the properties of logarithms.

First, we can simplify the equation by using the property that log a (b) = x is equivalent to a^x = b.

So, the equation becomes:

√3^4 * 5^21 = 3

Next, we can simplify the left side of the equation by expanding √3^4 and 5^21:

√81 * 9765625 = 3

Now, we can simplify further:

9 * 9765625 = 3

Finally, we can multiply 9 by 9765625 to get the final answer:

87890625 = 3

Therefore, the solution to the equation is 87890625.