3. Select all equations that can result from adding or subtracting these 2 equations: 5x+y = 12

3x - 5y = 4

a. -2x-4y=8

b. -2x+6y=8

c. 4x-4y=16

d. 4x+4y=16

e. 2x-6y=-8

f. 5x-4y=28

check the picture below.

well, the angle at the vertex d is clearly not a right-angle, let's check for the one at the vertex f which does look like,   however if it's indeed, that means ef is perpendicular to df, in which case their slopes will be negative reciprocal of each other.

so the negative reciprocal of the slope of ef is 1, however the slope of df is not 1, is 1/2, so nope, they're not perpendicular lines.

you can't solve x because it is 2 linear equation and you only have one equation