Answer
Trent's speed is 75 mph relative to Susan's reference frame.
Work Step by Step
Let $v_{Tg}$ be Trent's velocity with respect to the ground.
Let $v_{Sg}$ be Susan's velocity with respect to the ground.
Let $v_{TS}$ be Trent's velocity with respect to Susan.
$v_{Tg} = v_{TS}+v_{Sg}$
$v_{TS} = v_{Tg} - v_{Sg}$
We can find the east component of $v_{TS}$:
$v_{TS,east} = 45~mph - 0 = 45~mph$ (east)
We can find the south component of $v_{TS}$:
$v_{TS,south} = 0 - (-60~mph) = 60~mph$ (south)
We can find Trent's speed relative to Susan's reference frame:
$v_{TS} = \sqrt{(45~mph)^2+(60~mph)^2}$
$v_{TS} = 75~mph$
Trent's speed is 75 mph relative to Susan's reference frame.
