Answer:
Step-by-step explanation:
The slope formula is

Filling in this formula with 2 different points 2 times will ensure that this is fact a linear function. In other words, if the slope comes up the same between points 1 and 2 and then between points 2 and 3, this is linear.
Between (0, 600) and (3, 720) the slope formula is
 and
  and
 So this is in fact linear because the slopes are the same.
 So this is in fact linear because the slopes are the same.
Interpreting a slope is similar to interpreting a unit ratio. In our case, the slope means that after the initial investment in the account, $40 per day (there's the unit rate I was referring to) is deposited into the account.
Using that slope to write first the point-slope form of the line allows us to use any one of the 3 coordinates to write the equation. I use (0, 600). In point-slope form:

That's point-slope form. Now for slope-intercept form, which is the same as solving for y.
y - 600 = 40x - 0 and
y = 40x + 600. That's slope-intercept, and now for standard form. Standard form follows the format of a. No fractions and b. Both the x and y terms go on the same side of the equals sign and the constant on the other.
We move the x term over to the y side:
-40x + y = 600. But a lot of textbooks don't like to lead with a negative, so the equivalent form of this with a positive leading coefficient is
40x - y = -600 (notice that all we did was change ALL the signs to the opposite of what they were, not just the sign on the x term. Very important!!)