Similarity Theorems
Triangles ABG and DCB are similar by the AA similarity postulate
"If two angles in one triangle are congruent to two angles in another triangle, then the two triangles are similar"
The steps previous to statement 7 prove angles C and B are congruent because they are formed by two parallel lines that intersect a common line.
On the other side, angles DBC and AGB are congruent also because they are both right angles.
With two corresponding congruent angles, the AA similarity postulate is satisfied and the triangles are similar.