Question

without plotting find a point on the graph of x+y=21 whose y coordinate is three more than the x coordinate

Answer 1

y-coordinate is 3 more than x-coordinate:  y = x + 3.

Substitute to the equation x + y = 21:

x + x + 3 = 21      subtract 3 from both sides

2x = 18      divide both sides by 2

x = 9

y = 9 + 3

y = 12

Answer: (9, 12).