+2354 All right, so your question is;
$$1200 \times (1.05)^x = 3000$$
Do you know what a logarithm is?
Basically if we have
$$a^b = c$$
then
$$log_ac = b$$
There's a pretty good explanation about logarithms right here; http://www.mathsisfun.com/algebra/logarithms.html
Now let's rewrite your problem to $$1200 \times (1.05)^x = 3000 \Rightarrow (1.05)^x = \frac{3000}{1200} \Rightarrow x = log_{(1.05)}(\frac{3000}{1200})$$
Yet, we still have a problem...
Your calculator always uses $$log_{10}$$ if you use the $$log$$ button.
To be able to calculate this you need to make use of this rule;
$$log_ac = b \Rightarrow \frac{log_{10}c}{log_{10}a} = b$$
So now we have
$$x = \frac{log\frac{3000}{1200}}{log1.05}\Rightarrow x \approx 18.78$$
Let's check this;
$$1200*(1.05)^{18.78} = 2999.97 \approx 3000$$
Yay 
