The standard linear form is y=ax+b, where a=slope or gradient and b=y intercept (when x=0).
The given slope is 3/3.5=3*2/(3.5*2)=6/7.
To find b substitute y=-1 and x=-3: -1=6/7*(-3)+b. That is, -1=-18/7+b.
-7=-18+7b when we multiply through by 7. So 7b=18-7=11 and b=11/7.
Standard form: y=6x/7+11/7. (This can also be written less clumsily: 7y=6x+11.)