1200x1,05^x=3000

If your not yet studying logarithms, you can off course also make a table for some values of x.

You'd find that the 3000 is somewhere between x=18 and x=19. 

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