Question 455275
Since you have a binomial for the hypotenuse, make sure you square the entire binomial.  
Pythagorean theorem:  a²+b²=c²  where a and b are the legs and the c is the hypotenuse
Substitute your legs and hypotenuse into the formula and solve
8²+x²=(x+4)²
64+x²=x²+8x+16
64=8x+16  Subtract x² from both sides
48=8x  Subtract 16 from both sides
x=6  Divide both sides by 8
Now you can use x=6 to solve for the leg and hypotenuse.  
The leg is 6cm while the hypotenuse is 6+4, or 10cm.