You're learning about Pythagoras theorem right? We just finished that topic. so the formula is a^2+b^2=c^2. C is x in your case because it is the hypotenuse (the longest side in a right angled triangle). So just do 8^2+7^2=x. So it's 64+49=x^2
x^2= 113
So x = 10.6
