Answer
$$\color{blue}{(2a-p, 2b-q)}$$
Work Step by Step
To find the endpoint $C(x,y)$ when given the endpoint $D(p,q)$ and the midpoint $M(a,b)$ we will use the midpoint formula:
$${(\frac{x_{C}+x_{D}}{2},\frac{y_{C}+y_{D}}{2})}=M$$
$${(\frac{p+x}{2},\frac{q+y}{2})}=(a,b)$$
To find the $x$ value of $C$:
$\frac{p+x}{2}=a$
$p+x=2a$
$x=2a-p$
To find the $y$ value of $C$:
$\frac{q+y}{2}=b$
$q+y=2b$
$y=2b-q$
So, the endpoint is at:
$$\color{blue}{(2a-p, 2b-q)}$$
