Jason T. answered • 06/12/13

Jason Tutors Math & Physics

One way to tackle this problem is to graph the points to determine the relationship between y and x. If we write the coordinate pairs we have (0, 7), (1, 8), (2, 9), (3, 10), (4, 11). When you plot those you will notice a linear relationship.

To get the equation of the line, which will be the function rule, find the y-intercept and the slope. Remember that the y-intercept is where the line crosses the y-axis, so x = 0. One of our points is (0, 7). x = 0 and y = 7. **We will use 7 for the y-intercept in the equation.**

To find the slope we need the coordinates of any two points on the line. Lets use (0, 7) for point 1 and

(3, 10) for point 2. We could use any two points and, hopefully, get the same calculated slope. The formula for slope is (y2-y1)/(x2-x1).

Point 1 is (0, 7) so x1 = 0 and y1 = 7. Point 2 is (3, 10) so x2 = 3 and y2 = 10. Now substitute into the slope formula --> (10 - 7) / (3 - 0) = 3/3 = 1. So** slope = 1**

Now write the equation of the line in y = mx + b form. m is the slope and b is the y-intercept.

So y = 1x + 7. Now the 1 can be "invisible" so we normally write this **y = x + 7.** This is the rule, the equation of the line.

To find the values for y when x = 8, just substitute for x in the equation above:

y = 8 + 7 = 15 when x = 8, y = 15

When y = 35, substitute 35 for y and then solve for x

35 = x + 7

Subtract 7 from both sides --> 28 = x when y = 35, x = 28