Question

Solving systems by substitution (algebra 2)
7x + y = 12
y = 4x - 21​

Answer 1

we already know what y is so
7x+4x-21=12

add the common variables

11x-21=12

add 21 to 12 and -21

11x=33

divide 11 to 33 and itself

x=3

Answer 2

x = 3, y = -9

Explanation:

plug y=4x-21 into 7x+y=12 to get

7x + (4x-21) = 12 -> 11x -21 = 12 -> 11x = 33 -> x = 3

plug x=3 into y=4x-21 to get

y = 4(3) - 21 = 12 - 21 = -9