We are given that the two lines are parallel to each other. And we know that if two lines are parallel to each other then their slopes are equal. As the slope of green line is -3 , the slope of red line will also be -3.
The slope of any line represents the tangent of the angle which the line makes with the positive direction of x axis. As both the lines are parallel, they have same slope which is -3
A linear equation has 1 for all exponents on the placeholders aka the equation is first degree (aka the hightest exponent on placeholders=1) so it is linear
