Given the points ( 3 , 0 ) and ( 0 , -7)
The slope intercept form of the line y = mx + c
Where m is the slope and c is the y-intercept
The slope will be calculated as following:

so, y = (7/3) x + c
C is the value of y when x = 0
So, using the point (0, -7)
when x = 0 , y = -7
so,
c = -7
so,
y = (7/3) x - 7
Multiplying all terms by 3
so,
3y = 7x - 21
so,
3y - 7x = -21
so, the standard form of the line :
3y - 7x = -21 or 7x - 3y = 21