Answer
Option C is correct.
Explanation
We are told that the equation of a straight line is
y = 3x + 9
We are then told that if the 9 is replaced with a 3, what does this indicate?
When graphs are shifted upwards or downwards along the y-axis, the effect of this translation is shown in the new equation for the graph.
If a graph, y = f(x) is moved a units upwards on the y-axis, the new graph has an equation of y = f(x) + a
If a graph, y = f(x) is moved b units downwards on the y-axis, the new graph has an equation of y = f(x) - b
So, when a graph of y = 3x + 9 changes to y = 3x + 3, it indicates that 6 units have been subtracted from the original function; showing that the original function has been moved 6 units downwards.
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