First, you should think of things in fractions. 0.2 is a nasty way to think about the 1/5 because fractions were made to make things like multiplication easier. So let's rewrite this:
(1/5x +3)(4x -3/10)
From here, you can cross multiply. This means that we will be taking 1/5x and multiplying it across to the other parentheses containing (4x - 3/10). We will do the same with our +3 in the first parentheses. After that is done, we've finished.
So, first things first, 1/5x xx 4x which is 4/5x^2 as the 4 moves on top and get's divided by 5, and the variable, x, gets doubles making x^2. Then, 1/5x xx -3/10 which gives us -3/50x because the fractions simply multiply across, 1xx3 and 2xx10. Then all you have left to do is the same with the 3.
You will find that the answer is
4/5x^2 -3/50x + 12x - 9/10
From here, we just want to simply our like terms, meaning the single x's. Then you'll have:
4/5x^2 + 597/600x -9/10
We can express this in a different way. Since the original question used decimals, we can write the answer as:
0.8x^2 + 0.995x -0.9