The expression $x^2 + 18x - 63$ can be written in the form $(x - a)(x + b)$, where $a$ and $b$ are both nonnegative real numbers. What is the value of $b$?
I want to find two numbers $a$ and $b$ such that $(x - a)(x + b) = x^2 + 18x - 63$. This means that $-a + b = 18$ and $-ab = -63$, since these are the coefficients of $x$ and the constant term in the expansion of $(x - a)(x + b)$. I can use the first equation to eliminate one of the variables and substitute into the second equation. Then I can use the first equation again to find $b$. I get $b = 18 + a = 18 + 3 = 21$. So the value of $b$ is $21$.

