Financial Mathematics missing step for solution

Question

I got the first line, but I got lost when they moved to the second step. Can someone please explain to me how they moved from the first to the second line? This I am aware of is just Algebra, but if you need additional information be free to ask.

Answer 1

Expand: $$(1+j)^2=1+2.1j$$ $$1+2j+j^2=1+2.1j$$ Take everything to one side: $$j^2-0.1j=0$$ Factorize: $$j(j-0.1)=0$$ Hence: $j=0$ or $j=0.1$

In the scenario given there must be a reason for $j\neq0$ hence it is excluded.