Answer:
√(45x²-72x+53)
Step-by-step explanation:
When you insert diagonals, two rightangled triangles are formed.
To find the length of the diagonals, we put the rectangles into pythagorus's theorem:
a²=b²+c²
a²=(6x-7)²+(3x+2)²
a²=36x²-84x+49 + 9x²+12x+4
a²=45x²-72x+53
a=√(45x²-72x+53)
