What is the y-intercept of the graph of f(x) = x² - 8x + 21?

The $y$-intercept is where the graph crosses the $y$-axis, which happens at $x = 0$. Compute $f(0) = 0^2 - 8(0) + 21 = 21$. In standard form $ax^2 + bx + c$, the intercept is always the constant term $c$, so you can read it directly. A common wrong answer is 4, the $x$-coordinate of the vertex, $-\frac{b}{2a} = 4$. Another is 5, the $y$-coordinate of the vertex, since $f(4) = 16 - 32 + 21 = 5$. A third is $-8$, the coefficient of $x$. The intercept is not the vertex.