Let's first take a look at what happens in the first years.
Emily starts out with a salary of $25,000
After one year her salary has increased with 6%.
This means her salary is 106% of what it was last year.
Therefore her salary is now
$$(\frac{106}{100})\times\$25,000 = 1.06 \times\$25,000 = \$26,500$$
One year later her salary again increases with 6%
The means her salary is again 106% of what it was last year
Therefore her salary is now
$$(\frac{106}{100})\times\$26,500 = 1.06 \times \$26,500 = \$28,090$$
Now suppose we had the same situation and we wanted to know what her salary was after 2 years.
Instead of first calculating her salary after one year we could also have immediately calculated
$$(\text{salary after two years}) = 1.06 \times (\text{salary after one year}) = 1.06 \times 1.06 \times (\text{starting salary}) = (1.06)^2 \times(\text{starting salary}) = (1.06)^2 \times \$25,000 = \$28,090$$
Similarly, we could show that for 3 years we have that
$$(\text{salary after three years}) = 1.06^3 (\text{starting salary}) = 1.06^3 \times \$25,000$$
and that for x years we have that
$$(\text{salary after x years}) = 1.06^x = (\text{starting salary})$$
Now, if we rewrite the exercise as an equation we need to solve
$$1.06^x \times \$25,000 = 2 \times \$25,000$$
dividing both sides by 25000 gives us
$$1.06^x = 2$$
Now, I'm not sure what your level is so I'll give you two options
option 1 (the easy option (for whole years)):
Start with 1 and keep multiplying by 1.06 until the value is bigger or equal to 2.
The number of times you multiplied is the value of x.
To illustrate, this gives
$$\begin{array}{lcl}
1*1.06 = 1.06\\
1*1.06*1.06 = 1.06^2 = 1.1236\\
1*1.06*1.06*1.06 = 1.06^3 = 1.191016\\
(...)\\
1.06^{11} = 1.898298558\\
1.06^{12} = 2.012196472\\
\end{array}$$
Hence after 12 years, her income has doubled.
Option 2 (the advanced option (for partial years)):
This option makes use of logarithms. If you've never heard of logarithms, this method is not for you.
We indicate the natural logarithm as $$ln() = {}^elog()$$
We have that
$$\begin{array}{lcl}
1.06^x = 2\\
e^{ln(1.06^x)} = 2\\
e^{x \times ln(1.06)} = 2\\
x \times ln(1.06) = ln(2)\\
x = \frac{ln(2)}{ln(1.06)} \approx 11.8957
\end{array}$$
Hence the answer is approximately 11.8957 years
Reinout