Question 259800
You almost had it but this (3,0)=21 means nothing.
y=mx+b
0=3*-7+b
21=b
y=-7x+21
Perhaps you meant that the y intercept (b) =21

y=-7x+21 is in slope intercept form 
Standard form is ax+by+c=0

subtract y from both sides
0=-7x-y+21
-7x-y+21=0