Idon't know how to solve this!

what is the value of m in the figure below? in this diagram, δabd ~ δbcd.

To answer the question above, we make use of the Pythagorean theorem which can be mathematically expressed as,
                                 a² + b² = c²
There are 3 right triangles in the figure. ΔABC, ΔADB, and ΔBDC. The Pythagorean equations are,
                 ΔABC:   (AB)² + m² = (11 + 7)²
                 ΔADB:   11² + (BD)² = (AB)²
                 ΔBDC:  (BD)² + 7² = m²

We may use either substitution or elimination method to solve for AB, BD and m. The answer I got is 11.22. 

I’m pretty sure the answer is a

9.8? i'm not really sure