$$ 3 i s i n = 3 i^{1 + 1} n s = 3 i^2 n s $$

Use the distributive property.

Multiply $p$ by $ \dfrac{i}{3} $ to get $ \dfrac{ ip }{ 3 } $.

Step 1: Write $ p $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} p \cdot \frac{i}{3} & \xlongequal{\text{Step 1}} \frac{p}{\color{red}{1}} \cdot \frac{i}{3} \xlongequal{\text{Step 2}} \frac{ p \cdot i }{ 1 \cdot 3 } \xlongequal{\text{Step 3}} \frac{ ip }{ 3 } \end{aligned} $$

Multiply $3cos+3i^2ns$ by $ \dfrac{-ip}{3} $ to get $ \dfrac{-3i^3nps-3ciops}{3} $.

Step 1: Write $ 3cos+3i^2ns $ as a fraction by putting $ \color{red}{1} $ in the denominator.

Step 2: Multiply numerators and denominators.

Step 3: Simplify numerator and denominator.

$$ \begin{aligned} 3cos+3i^2ns \cdot \frac{-ip}{3} & \xlongequal{\text{Step 1}} \frac{3cos+3i^2ns}{\color{red}{1}} \cdot \frac{-ip}{3} \xlongequal{\text{Step 2}} \frac{ \left( 3cos+3i^2ns \right) \cdot \left( -ip \right) }{ 1 \cdot 3 } = \\[1ex] & \xlongequal{\text{Step 3}} \frac{ -3ciops-3i^3nps }{ 3 } = \frac{-3i^3nps-3ciops}{3} \end{aligned} $$