Add MathJax to Linear Algebra examples

This makes it clear what linear equations we're actually solving, so the reader doesn't have to try and work that out at the same time as trying to comprehend how the code goes about doing this.

README.md

@@ -27,7 +27,15 @@ using (var ctx = new Z3Context())
 
 ### Problem - Linear Algebra
 
-Solve the following system with 3 variables, with linear equalities and inequalities.
+Solve the following system with 3 variables, with linear equalities and inequalities:
+
+$$
+x_1 - x_2 \ge 1
+\\
+x_1 - x_2 \le 3
+\\
+x_1 = 2x_3 + x_2
+$$
 
 ```csharp
 using (var ctx = new Z3Context())

examples/z3-problems.dib

@@ -62,6 +62,14 @@ using (var ctx = new Z3Context())
 
 Solve the following system with 3 variables, with linear equalities and inequalities.
 
+$$
+x_1 - x_2 \ge 1
+\\
+x_1 - x_2 \le 3
+\\
+x_1 = 2x_3 + x_2
+$$
+
 #!csharp
 
 using (var ctx = new Z3Context())