ANSWER

EXPLANATION
The given polynomials are:

and

We want to subtract these two polynomials.

Expand the right hand side to obtain:

Group the similar terms:

Combine the similar terms to get:

After 4 years, the account gains $325.8, so 4 years
In order to determine whether the equations are parallel, perpendicular, or neither, let's simply each equation into a slope-intercept form or basically, solve for y.
Let's start with the first equation.

Cross multiply both sides of the equation.


Subtract 6x on both sides of the equation.


Divide both sides of the equation by -5.


Therefore, the slope of the first equation is 4/5.
Let's now simplify the second equation.

Add x on both sides of the equation.


Divide both sides of the equation by -4.


Therefore, the slope of the second equation is -5/4.
Since the slope of each equation is the negative reciprocal of each other, then the graph of the two equations is perpendicular to each other.
Write the equations,
10 c + 5 f = 6900
5c + 3f = 3660
Double,
10c + 6f = 7320
Subtract,
f = 7320 - 6900 = 420
10 c = 6900 - 5f = 6900 - 2100 = 4800
c = 480
Answer: Cheeseburger 480 calories, fries 420 calories
Check:
10 (480) + 5 (420) = 4800 + 2100 = 6900 good
5(480) + 3(420) = 2400 + 1260 = 3660 good