feat: complete AP Physics C handbook — all E11-E13 concepts, unit assemblies, and fixes

concepts/em/u11/e11-5-kirchhoff.tex

@@ -138,7 +138,6 @@ Substitute equation (1) into equation (2):
 \[
 7.0\,I_1 + 3.0\,I_2 = 12.
 \]
-\label{eq:a}
 
 Substitute equation (1) into equation (3):
 \[
@@ -158,9 +157,8 @@ so
 \[
 I_1 = 2.0 - 3.0\,I_2.
 \]
-\label{eq:b}
 
-Substitute equation (\ref{eq:b}) into equation (\ref{eq:a}):
+Substitute equation (3) into equation (2):
 \[
 7.0\,(2.0 - 3.0\,I_2) + 3.0\,I_2 = 12,
 \]
@@ -177,7 +175,7 @@ Substitute equation (\ref{eq:b}) into equation (\ref{eq:a}):
 I_2 = \frac{2.0}{18.0} = \frac{1}{9}\,\mathrm{A}.
 \]
 
-From equation (\ref{eq:b}):
+From equation (3):
 \[
 I_1 = 2.0 - 3.0\left(\frac{1}{9}\right) = 2.0 - \frac{1}{3} = \frac{6}{3} - \frac{1}{3} = \frac{5}{3}\,\mathrm{A}.
 \]