Question

I don't understand these problems

Answer 1

So given the values on the number line, AB and BC, adding them together will get the length of AC. If AB equals $x-3$ and BC equals $5$, the value of AC should be $x-3+5$. Since you are given the length of AC, $21$, you can create an equation to solve for x. This equation would be $x-3+5=21$. Isolating the x on the left (or right hand side) gets the value of x to be 19. The same thing goes for the 2nd question.