Question

Which point is a solution of y = 3x and x + 2y = –21?
Select one:
a. (–3, –9)
b. (–9, –3)
c. (3, –9)

Answer 1

just substitute each of the coordinate in both equations to see which one works out

Answer 2

As AriPotta is saying


x + 2y = –21

x + 2(3x) = –21 ... replace y with 3x (works because y = 3x)

x + 6x = -21

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x = ???