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.