First we must look at the sequence 15,9,3,-3...
You can see that the value is decreasing by 6 every term. Because the rate is drawing at a constant rate the sequence must be linear, meaning that the slope is -6.
If we look at the general function of a line in slope intercept form:
where m is the slope and b is the y intercept. The trick to find the y intercept is to go backwards, in other words what was the previous term? 15+6=21. Therefore, m=-6 and b=21 and thus,
Since we want to know the value of y even x=20 we obtain:
You can see that the value is decreasing by 6 every term. Because the rate is drawing at a constant rate the sequence must be linear, meaning that the slope is -6.
If we look at the general function of a line in slope intercept form:
where m is the slope and b is the y intercept. The trick to find the y intercept is to go backwards, in other words what was the previous term? 15+6=21. Therefore, m=-6 and b=21 and thus,
Since we want to know the value of y even x=20 we obtain: