Calculate the length of the altitude through A of an isosceles triangle in which AB=AC=26 cm and BC=20 cm.

The straight line down from the tip to the middle of the base makes a right triangle with the little bottom side measuring 10 and the hypotenuse, the angley side, measuring still what it is, 26.  Bummer it's not a 3, 4, 5 triangle, but anyway, all you have to do is plug the numbers into the Pythagorean theorem and solve.  But 10 and 26 are pretty big numbers, so I'd first divide both by two, making them 5 and 13 and then solve the problem 13 squared = 5 squared + what squared? 

169 = 25 + x squared

169 -25 = x squared

144 = x squared (o thank gosh, an easy square!)

so x = the square root of 144, 12.

But then remember to multiply it back up (because you halved the sides to make getting the answer easier). 

So 12 x 2 = 24.

So the altitude is 24.

Now I'm going to see if 24's what the other answerer got, just a sec . . .

Yes!  cool.  not to shabby for an English teacher, eh?