Ok. The equation of a line can be written as y = mx + c where c is the y-intercept and m is the gradient (slope).
So, once you have c and m, you can form the equation.
Let's look at the first graph.
1. What is the y-intercept, ie the value where the line cuts the y-axis. This is the value of "c".
2. What is the gradient of the line? For this you will need to read off 2 points on the graph. One is (0, c). Find another point which is easy to read off the graph (ie preferably where the grid lines meet). Then find the gradient between these two points. (Do you recall the formula for gradient?)
This will give you the value of "m".
3. Substitute your values for c and m into y=mx+c and that is the equation you want.
(Note: Depending on where you are from, you may have learnt this as y = mx+b. Same deal.)
Show us your answer for Question 1 so that we can see you are on the right track..