If two vectors are perpendicular to each other, how should you add them?

use the pythagorean theorem

Let us consider two vectors A and B.

As per the question, the two vectors are perpendicular to each other.

Hence the angle between them  

We are asked to calculate the resultant of these two vectors.

As per parallelogram law of vector addition, the resultant of two vectors are-

                                   [cos90=0]

This is the way by which we can add two perpendicular vectors.

force is mass*acceleration.

we have a very small mass: 11.4*0.001 kg

if we want 0.035n of force, we need an acceleration:

0.035 = 11.4*0.001*a

a = 0.035/(11.4*0.001) m/s^2

hypothesising or postulating