In the mentioned step(from line 2 to line 3), (2x) on the right hand side of the equal sign is transferred to the left hand side.
What is a linear equation?
A linear equation is an equation that has the highest power of the variable of 1.
Given linear equation is:
3x + 9 - 7x = x + 10 + x
⇒ - 4x + 9 = 2x + 10
Here, (2x) is transferred from the right hand side to the left hand side.
⇒ - 6x + 9 = 10
Learn more about linear equation here: https://brainly.com/question/2742698
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