multiplicity is how many times the root (or zero) is repeated
example, in

the root b has a multiplicity of n and the root d has a multiplicity of m
first we need to get the roots into

for (3x+5), we need to force factor out the 3 to get

factor the other one

but it is

which is equivilent to

which simplifies to

so we get

so the roots are -5/3 multiplicity 1 and 3 multiplicity 4
3 has a multilicity of 4