Can someone explain me the equations on #

x + 2y = 12

x - y = 3

Add y to both sides of the second equation.

x = 3 + y

Plug this equation into the first equation.

3 + y + 2y = 12

Combine like terms.

3 + 3y = 12

Subtract 3 from both sides.

3y = 9

Divide both sides by 3.

y = 3

Now that we have the value of y, we can find the exact value of x.

Plug this value into the second equation.

x - 3 = 3

Add 3 to both sides.

x = 6

We can verify by plugging these values into either equation.

6 + 2(3) = 12

6 + 6 = 12

12 = 12

The values are correct.

it has to be a.

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.