For the equation y=7x+3 the value of x changes from 1 to 2

When x=1, 7x+3 = 10.

When x=2, 7x+3 = 17.

When the value of 'x' changes from '1' to '2', the value of 7x+3 changes
from '10' to '17' . . . a distance of '7' .

That's what the ' 7x ' in the equation means, and that's what we mean
when we say that the 'slope' of the equation is '7'.  'y' changes '7' for
every '1' that 'x' changes.

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