Question 79939
Think of (1) the ladder (2) the bit from the ground to the foot of the ladder and (3) the height of the ladder up the wall as making a triangle.
Draw yourself a little diagram if it helps.
Assume the angle between the ground and the wall to be 90 degrees, (it doesn't say in the question, but if it isn't then you can't solve it with just the amount of information given)
Now, if it is a right angled triangle then it will obey Pythagoras's law (you know, the square on the hypotenuse, etc etc. .....)

So if the height up the wall is h, then we can say h^2 + 7^2 = 26^2

h^2 = 26^2 - 7^2

so h^2 = 676 - 49

h^2 = 627

h = 25.04 feet (ANSWER)