Answer:
This is a right triangle, so we know that:
h² = b' · c'
which is this case can be specificly written as:
BD² = AD · CD
BD² = 7 · 3 = 21
BD = √21
Now that we can also notice that ΔADB is also a right triangle, therefore we can apply the pythagorean theorem:
AD² + BD² = AB²
7² + (√21)² = x²
x² = 49 + 21 = 70
x = √70
