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Answer: Choice C. -7x - 5y = -48</h3>
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First we need the equation of line AB. Compute the slope through (-3,-1) and (4,4)
m = (y2-y1)/(x2-x1)
m = (4-(-1))/(4-(-3))
m = (4+1)/(4+3)
m = 5/7
This is the slope through points A and B, ie the slope of line AB.
To get the slope of line BC, we flip the fraction and change the sign.
Doing so has us go from 5/7 to -7/5. Note how 5/7 and -7/5 multiply to -1.
The slope of line BC is -7/5. Let m = -7/5.
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Use (x,y) = (4,4) along with m =-7/5 to find the equation of line BC
y = mx+b
4 = (-7/5)(4) + b
4 = -28/5 + b
20 = -28 + 5b ... multiply every term by 5 to clear out the fraction
20+28 = 5b
48 = 5b
5b = 48
b = 48/5
With m = -7/5 and b = 48/5, we go from y = mx+b to y = (-7/5)x+48/5
The slope intercept form for line BC is y = (-7/5)x+48/5
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Let's get this into standard form Ax+By = C
y = (-7/5)x+48/5
5y = -7x + 48 .... multiply everything by 5
7x+5y = 48 .... is one way to represent the equation in standard form
-7x - 5y = -48 ... your teacher has decided (for some reason) to multiply both sides by -1
