I struggle with proving big oh, thus I wonder if (Kahn) or anyone else can help me prove that: t(n) <= cg(n) for all n >= 0 and here is the example I do not g

I struggle with proving big oh, thus I wonder if (Kahn) or anyone else can help me prove that: t(n) <= cg(n) for all n >= 0 and here is the example I do not grok, where I'm gonna prove this assertion: 100n + 5 ∈ O(n^2). This proof goes like this: 100n + 5 <= 100n + n = 101n <= 101n^2 I struggle basically with wrapping my brain around the proof there. What is going on?