I agree with Suzyboo - what was the Q that was set by your daughter's tutor ?
A straight line graph is just a linear equation expressed in terms of x and y. For example, y = mx plus c where m is the gradient and c is the intercept (where it crosses the y axis) (and as per normal, x is the horizontal axis and y is the vertical axis.....)
As I can feel your eyes starting to glaze over, have a wee look at
this site. You can adjust m and c to see what effect they have when x=y. Hopefully that will clarify things!