What does it mean by change in x coordinate and change in y coordinate?

I am trying to find the gradient in a line graph in order to find g (the acceleration of gravity towards the earth) and i have got 19.739... which is miles off!!! i am thinking it must be my gradient i have got wrong because i have always been confused with it. so how do you work it out, step by step becausei dont understand what the change is - help!