Solve the following system of equations by linear combination: 3d – e = 7 d + e = 5

3d - e = 7d + e = 5   ⇒   3d - e = 5 
                                       7d + e = 5

 3d - e = 5 
 7d + e = 5          +

10d / = 10
10d = 10
d = 10/10
d = 1

 7d + e = 5
7×1 + e = 5
7 + e = 5
e = 5 - 7
e = -2

None of the options you showed are it.