Find the opposite length of the triangle with hypotenuse length 5 and adjacent length 4.

I would start off by asking them if they know the Pythagoras theorem. If they don't, I would refresh their memory and explain that its a²+b²=c² where c is the hypotenuse (longest length). I would then ask them to draw out the triangle to help them visualise it and ask them to label the sides with a, b or c. Following this, I would ask the student to substitute the values given in the question into the equation and rearrange it so a is in the front and then go on to solve the equation. 

So b=4 and c=5. a²+4²=5² hence a²=5²-4². a²=25-16=9. a²=9 so a=3

Once solved, I would ask them another question of the same topic and without prompts, ask them to solve it themselves to see where their understanding might be hindered. I would go through the steps with them again. 1) Draw the triangle 2)Label the sides with a,b or c and their values  3) enter the values into the equation a²+b²=c²  4)solve the equation