Hello!
To put -3x-7y-14=0 into standard form, or ax+by=c where the a value cannot be negative, and there are no fractions or decimals, or any common factors, first add -14 to both sides to get -3x-7y=14. Then, since your a value is negative, multiply the entire equation by -1 which will result in your answer:
3x+7y=-14
Hope this helps!