y = 2x - 7...the slope here is 2. A parallel line will have the same slope and a perpendicular line will have a negative reciprocal slope.
parallel line....
In y = mx + b form, the m is ur slope and the b is ur y int. A parallel line will have the same slope but will have a different y int. So one parallel line could be : y = 2x + 7.....or y = 2x - 4....that last number can be anything else but -7.
perpendicular...
perpendicular lines have negative reciprocal slopes. To find the negative reciprocal of a number, just flip the slope and change the sign. So the negative reciprocal of 2 (or 2/1) is - 1/2....see how I flipped the slope and changed the sign. So our perpendicular line will have a slope of -1/2. So one perpendicular line could be : y = -1/2x + 5...or y = -1/2x - 2.....doesn't really matter what the last number is, just as long as ur slope is -1/2.