So this is a right angled triangle so you can use Pythagoras theorem for it (a^2 + b^2 = c^2) and c^2 is always the hypotenuse so you just substitute that in as 8^2 + x^2 = 17 ^2 and then square the values you know so 64 + x^2 = 289 and then do 289 - 64 = x^2 so x^2 = 225 and then you square root 225 to get 15 so you know x is 15 cm
