3 kg + 4 kg always add up to be 7 kg. But 3 N + 4 N can be anything from 1 N to 7 N, depending on the direction of the two forces.
To add or subtract two vectors, you can use the so-called triangle method, where the resultant vector is the side that closes the triangle.
Alternatively, you can use the so-called parallelogram method, where the resultant vector is the diagonal of the parallelogram.
After sketching the vector diagram, you can solve for the magnitude and direction of the resultant vector mathematically, typically using cosine rule or sine rule.
Two Perpendicular Vectors
The calculation is even easier if the two vectors are perpendicular to each other. Since the resultant vector is the hypotenuse of a right angled triangle, all you need is Pythagoras and trigo ratios.
Two Equal Magnitude Vectors
If the two vectors of the same magnitude, then the resultant vector is the diagonal of a rhombus, which again makes the calculation pretty simple.