The top is a triangle with sides 7 and 5.4 Use the Pythagorean theorem to find the 3rd length.
3rd sides = sqt(7^2 + 5^2( = sqrt(49+25) = sqrt(74)
This side is the base of the lower rectangle.
The dotted line is the hypotenuse of a right triangle.
Use the Pythagorean theorem to solve.
sqrt(10^2 + sqrt(74)^2) = sqrt(174)
The dotted line = sqrt(174)