The line whose equation is x = 7 is this green vertical line:



It has no slope and no y-intercept, thus it cannot be written

in the slope-intercept form y=mx+b

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The line whose equation is y = 4 is this green horizontal line:



It has slope = 0 and y-intercept (0,4). It can be written

in the slope-intercept form 

y = mx + b 

as

y = 0x + 4

Edwin