Those steps show we start with the outer function f(x) = 5x-3. Then we replace every x with g(x) in the second step. Afterward, we plug in g(x) = 6x^2-3 and we simplify.
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We'll follow similar steps for (g∘f)(x)
But this time we started with g(x), plug in f(x) for every x, and then simplified.