With these questions it is asking you to take a common factor out of each equation.
Let’s take the second question: 19x³ - 38x²
We need to find something that will go into 19x³ and 38x²
First off, they both contain x so we can divide them by x. We take the x out and put the rest in brackets, like this:
x(19x² - 38x)
They still have x so we can do that again.
x²(19x - 38)
Now, does anything go into 19x and 38? Yes, they are both divisible by 19. So take that out too!
19x²(x - 2)
Nothing more goes into x or 2, so the problem is solved!
Remember to always take a common factor out before you try to factorise. (So try question 15 again with this in mind)
You can also check your answer by expanding it again.