Would you please help me with number 8 on page 56 please?

    \begin{align*}\dfrac{pqr}{mn}+34a=374\end{align*}

Sure. You’re starting with the equation above and your mission is to solve it for a. First, move the fraction over by subtracting it from both sides:

    \begin{align*}\dfrac{pqr}{mn}+34a&=374\\\\34a&=374-\dfrac{pqr}{mn}\end{align*}

Then divide by 34 (or, in other words, multiply by \dfrac{1}{34}):

    \begin{align*}\dfrac{pqr}{mn}+34a&=374\\\\34a&=374-\dfrac{pqr}{mn}\\\\a&=\dfrac{1}{34}\left(374-\dfrac{pqr}{mn}\right)\\\\a&=\dfrac{374}{34}-\dfrac{pqr}{34mn}\\\\a&=11-\dfrac{pqr}{34mn}\end{align*}