By the general application of cumulative property of addition :
x + y = y + x
For sure
bearing in mind that standard form for a linear equation means
• all coefficients must be integers, no fractions
• only the constant on the right-hand-side
• all variables on the left-hand-side, sorted
• "x" must not have a negative coefficient

![\bf \begin{array}{|c|ll} \cline{1-1} \textit{point-slope form}\\ \cline{1-1} \\ y-y_1=m(x-x_1) \\\\ \cline{1-1} \end{array}\implies y-7=1[x-(-1)]\implies y-7=x+1 \\\\\\ y=x+8\implies \boxed{-x+y=8}\implies \stackrel{\textit{standard form}}{x-y=-8}](https://tex.z-dn.net/?f=%5Cbf%20%5Cbegin%7Barray%7D%7B%7Cc%7Cll%7D%20%5Ccline%7B1-1%7D%20%5Ctextit%7Bpoint-slope%20form%7D%5C%5C%20%5Ccline%7B1-1%7D%20%5C%5C%20y-y_1%3Dm%28x-x_1%29%20%5C%5C%5C%5C%20%5Ccline%7B1-1%7D%20%5Cend%7Barray%7D%5Cimplies%20y-7%3D1%5Bx-%28-1%29%5D%5Cimplies%20y-7%3Dx%2B1%20%5C%5C%5C%5C%5C%5C%20y%3Dx%2B8%5Cimplies%20%5Cboxed%7B-x%2By%3D8%7D%5Cimplies%20%5Cstackrel%7B%5Ctextit%7Bstandard%20form%7D%7D%7Bx-y%3D-8%7D)
just to point something out, is none of the options, however -x + y = 8, is one, though improper.
Substitute y = 3x + 15 to the equation -4x + 7y = 20:
-4x + 7(3x + 15) = 20 <em>use distributive property</em>
-4x + (7)(3x) + (7)(15) = 20
-4x + 21x + 105 = 20 <em>subtract 105 from both sides</em>
17x = -85 <em>divide both sides by 17</em>
x = -5
Substitute the value of x to the equation y = 3x + 15:
y = 3(-5) + 15
y = -15 + 15
y = 0
<h3>Answer: x = -5 and y = 0</h3>
x = 0, x =
, x = - 
since we have a product of factors equal to zero, equate each factor to zero and solve for x
x² = 0 ⇒ x = 0 ( multiplicity 2 )
5x - 7 = 0 ⇒ x = 
3x + 2 = 0 ⇒ x = - 