I really dont understnd this vector problem. please help!

In this question,iandjare unit vectors in the direction of the positive x- and y-axes respectively.

Given thata = i – 2jandb = –3i + jfind

(a)λ,ifa + λbis parallel to– i – 3j

(b)μ,ifμa + bis parallel to2i + j.

in the solution bank it shows the answer worked out like this but I just dont get the working.

is parallel to if

I dont understand how they got this expression ?

This is parallel to – i – 3j if