Answer
$d=a-b$ where $d$ is the amount of check that you forgot to record, $a$ is the balance according to your thought and $b$ is the actual balance.
$\implies d=68\$-26\$=42\$$
Work Step by Step
Let the amount of check that you forgot to record be $d$. Let the balance according to your thought be $a$ and the actual balance be $b$. Then $d=a-b=68\$-26\$=42\$$
