The slope is defined in the equation by the number before the x and the y-intercept is defined in the equation by the number that follows. If there is no number then the y-intercept is zero.
when dina arrived to 20 masha was at 16. so the relation is 20/16 and if dina's speed was 10 your formula is 10/x=20/16 that means x/10=16/20. so x will be 8