Why can you use cross products to solve the proportion StartFraction 18 over 5 EndFraction = StartFraction x over 100 EndFraction for x?

We first find the slope of the line that passes through those points. The slope formula tells us that the slope of a line that passes through points and is . We let be and be . So, now we have which is . Now can use the slope formula again to make an equation. We know that is  in this case. We let be a point we know. Let's just use . So we know that Now we keep on simplifying this to get . We keep on simplifying to get