Can someone explain how this problem is solve? i'm not understanding how they came up with 4.

Since the denominator is 9 and they multiplied it by 4, the numerator;5 also needs to multiply by 4. 

Same thing with this: If the denominator is 9 and they multiplied by 8, the numerator is also must be multiplied by 8

It's 4 because 9 times 4 equals 36

Ifound this on google “that means that the graph of a function with direct variation has these features: it is a straight line: because the slope is the ratio y/x = k. it passes through the origin (0,0): because y = kx = k (0) = 0.” i hope this you for what your looking for