fix some problems

src/7_differential_equations_1.md

@@ -790,7 +790,7 @@ $(A- \lambda I)\vec{v}_{i} = \vec{v}_{i-1}$. This completes the proof
 since we have demonstrated that $\vec{\phi}_{k}(t)$ is a solution of the DE.
 
 
-# Problems
+## Problems
 
 1. [:grinning:] Solve:
 
@@ -907,8 +907,8 @@ since we have demonstrated that $\vec{\phi}_{k}(t)$ is a solution of the DE.
 
 9. [:sweat:] Find the general solution of the system of equations
 
-    $$\dot{x_1} = 3 x_1 + x_2$$
-    $$\dot{x_2} = - x_1 + x_2$$  
+    $$\dot{x}_1 = 3 x_1 + x_2$$
+    $$\dot{x}_2 = - x_1 + x_2$$  
 
 10. [:sweat:] *Bonus question - A question of this kind will not be on the exam*