We will have the following:

So, the heat to add is 3611.52 Joules.
1,000 W = 1 kW
100 W = 0.1 kW
(0.1 kW) x (6 h) = 0.6 kWh <=== energy
(0.6 kWh) x (£0.1359/kWh) = £0.0815 <=== cost of it
Answer:
Explanation:
base of triangular frame, b = 90 cm
Area, A = 765 cm²
Let the height is h.
Area of a triangular frame = 1/2 x base x height
765 = 0.5 x 90 x h
h = 17 cm
Thus, the height of triangular frame is 17 cm.