The population of california is approximately 38,040,000. the population of texas is about 2.606 × 10^7. what is the approximate total population of these two states?

With scientific notation remember this rule; if the exponent is positive, move the decimal point that many places to the right. If it is negative move it that many places to the left. What that will do is make the number larger or smaller to the correct value.
 
So for example, if you had to solve 1.02 x 10^4 you would move the decimal four places to the right. So you would end up with 10200. 
As another example if you had to solve 1.02 x 10^-2 you would move the decimal two places to the left. That would give you .0120. 

So now we are onto your problem. 2.606 x 10^7.
You would move the decimal seven places to the right, because the exponent is positive, that will make your number larger(which would make more sense if you were adding populations, right?).
That would give you 26,060,000 people living in Texas.
From there simply add the two populations together and you will have your answer!