A5m long ladder is placed against a wall with its foot 1.8m from the wall. find how far up the wall the ladder reaches?

Since you have the hypotenuse (5) and the adjacent side (1.8) all you have to do is find out what squared times 1.8 squared equals 5 squared. the formula for this is a^2 + b^2 = c^2

Now to figure this out you can do 5^2 which equals 25. 
Then you do 1.8^2 which equals 3.24. then you subtract the two and get 21.76.
if you find the square root of 21.76 you will get 4.6647.
4.6647 is the distance of how far up the wall the ladder reaches.