You should use slope-intercept form on graphs like this.
Slope-intercept form looks like this:
To use slope-intercept form, you must use 2 points that are located on the graph.
Points on a graph look like (x,y), with x being the first number in the parenthesis and y being the second number.
___________________________________________________
For the first graph you would use the points (15,20), which would be your x2 and y2, and (13,12), which would be your x1 and y1.
Now you subtract 12 from 20 to get 8, and 13 from 15 to get 2.
Divide 8 by 2 to get your slope, which is your Rise over Run.
8(Rise) over 2(Run) equals 4(Rise) over 1(Run).
So, now that we've got your slope, you must use it to find the missing x value for #1.
The missing x value for the first graph would be 29.