Erin has a balance of -$17 in her bank account and Kathy has a balance of $23. What is the difference between the two account balances?

What the question is really asking

“Difference” between two balances means how far apart the amounts are on the number line, not who has more money (though Kathy does).

Subtract the balances (watch the negative)

Compute the difference by subtracting Erin’s balance from Kathy’s balance:

$$23 - (-17) = 23 + 17 = 40$$

Quick number line check

From $-17$ up to $0$ is $17$, and from $0$ up to $23$ is $23$. Together, $17 + 23 = 40$, so the balances are $40 apart.