A = (1/2 * L) * h
You want to find 1/2 * L, so divide both sides by h.
A/h = 1/2 * L
40/4 = 10cm = 1/2 * L.
Hope the solution helped. Sometimes you need more than a solution. Contact fcabanski@hotmail.com for online, private tutoring, or personalized problem solving (quick for groups of problems.)