When solving a question for geotech PE exam preparation, I saw a question that gave the weight of a structure and the spectral acceleration and asked for the base shear. (question number 95 in six minute solution for geotech PE book)

They used the formula of

V=W x Sa / g

W is weight, Sa is spectral acceleration and V is base shear...

of course i understood the formula but i didnt find the theory to study about this. can you explain or refer me to somewhere to understand why is this so?