Does the point (25,75) lie on a straight line y=3x. how do you know that

The answer is yes!

you know this because when you look at the point (25,75) you should automatically think (x,y) where "x" is what you plug in to the line to get "y" which is your output.

so that being said if you plug in 25 ...

y= 3(25)

you want your answer to be 75. if it is then this point exist on the line if it isnt then it does not exist on this line.
 
in this case 3*25 does in fact equal 75 so the point (25,75) exist on the line y=3x