Answer :
3x+4y=12
4y=12-3x
y=(12-3x)/4 <---------- This is f(x)
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3x+4y=12
3x=12-4y
x=(12-4y)/3
Therefore:
f⁻¹(x)=(12-4x)/3
=[4(3-x)]/3
4y=12-3x
y=(12-3x)/4 <---------- This is f(x)
---------------------
3x+4y=12
3x=12-4y
x=(12-4y)/3
Therefore:
f⁻¹(x)=(12-4x)/3
=[4(3-x)]/3