#### Answer

$x = -\frac{5}{2}$
The student added $3 + 5$ together to obtain $8$. This is where Frank should have treated $\log_2 (x+3)$ as separate from $5$. He should not have added them together. Instead, Frank should have subtracted $5$ from both sides. (Refer to correct working).

#### Work Step by Step

$\log_2 (x+3) + 5 = 4$
$\log_2 (x+3) = 4 -5$
$\log_2 (x+3) = -1$
$2^{-1} = x+3$
$0.5 - 3 = x$
$x = -\frac{5}{2}$
The student added $3 + 5$ together to obtain $8$. This is where Frank should have treated $\log_2 (x+3)$ as separate from $5$. He should not have added them together. Instead, Frank should have subtracted $5$ from both sides (Refer to correct working).