It appears you've uploaded a document containing multiple mathematical and chemical tasks. I'll begin by extracting relevant details and solving each step by step. Let me analyze the image content and offer precise solutions to the questions posed.

Problems Extracted:

1. Câu 1 (0.5 điểm):
   Solve the system of differential equations:
   $\frac{dy_1}{dt} = 2.1y_1 + 1.5y_2 + 0.6t$
   $\frac{dy_2}{dt} = 1.8y_1 + 2.1y_2 + 0.4t$
   Given initial values: $t=0, y_1^0=y_2^0=1$, with step size $h = 0.5$, solve for $t = [0, 1.5]$ (3 steps).

2. Câu 2 (0.5 điểm):
   Minimize the function:
   $f(x) = 1.75x_1^2 + 1.1x_2^2$
   within constraints $5 \leq x_1 \leq 6$ and $5 \leq x_2 \leq 6$ using the Davidson-Fletcher-Powell method (2 iterations).

3. Câu 3 (1.5 điểm):
   Analyze the chemical process diagram presented. The task involves evaluating mass balances across devices and analyzing ammonia (NH₃) concentration distributions across flows $Q_1, Q_2$. This requires qualitative assumptions, writing mass flow balances, and analyzing flows labeled $V_1, V_2, V_3, V_4, V_5$, considering initial chemical balance law assumptions.

Where would you like me to begin?

1. Solve Câu 1 (Differential equations step-by-step).
2. Solve Câu 2 (Constrained optimization problem).
3. Analyze Câu 3 (Flow chart and chemical reactions).

Feel free to direct me or request detailed explanations for all steps.