You wanna find a common denominator. An easy way to do that imo is to multiply the two denominators so here it would be 7 x 4. That would mean 28 is a common denominator. To find the numerators, it would be 6 x 4, and 1 x 7. So it would mean -24/28 + 7/28 which would end up being -17/28 which isn’t reducible.
