I would be happy to help but it seems there are no pictures attached to answer from.
This system of equations has no solution.
Given that B is the midpoint of line AC and line BC is congruent to line DE.
The following statements and reasons, proves that line AB is congruent to line DE.
Statement Reasons
1. B is the midpoint of line AC Given
2. Line AB is congruent to line BC. Midpoint of a line segment
3. Line BC is congruent to line DE Given
4. Line AB is congruent to line DE Transitive property

According to given figure, The Triangle BDC is a right angled Triangle (Angle D = 90°)
So, we can apply Pythagoras theorem to find the Side a (Hypotenuse)
Now, let's solve ~
Therefore, Length of side a is 5 units ~