(d) $-1, -3$
Factorising: $x^2 + 4x + 3 = (x+1)(x+3) = 0 \Rightarrow x = -1$ or $x = -3$.
Source: Chapter 2, Section 2.3
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Factorise by splitting the middle term: $4x = 3x + x$, giving $(x+1)(x+3)$. Setting each factor to zero gives both zeroes as negative. A common mistake is ignoring the signs — since all coefficients are positive, both zeroes must be negative, ruling out options (a), (b), and (c) immediately.