Dashed lines mean add, and solid lines mean multiply.

For example, two dashed lines point from ? and $\large 3$, so we add those numbers together to fill in the circle they point at.

What number must go in the yellow marked circle?

We can work backwards starting from the $\large 16$ we know the circles that lead to it multiply to be $\large 16$, and one of the values is $\large 2$. Since $\large 2 \times 8 = 16$ the other value has to be $\large 8$.
Now we know the bottom portion is $\large ? + 3 = 8$ the question mark needs to be $\large 5$ for the addition problem to work.
Incidentally, this process is equivalent to solving for $\large x$ in the equation $\large 2(x+3) = 16$. It's fairly natural in the charted form to work backwards from the $\large 16$, and this is exactly what needs to be done to solve when the equation is in algebraic form.