Respuesta :
remember, you can do anything to an equation as long as you do it to both sides
u=x-k
add k to both sides
u+k=x-k+k
u+k=x+0
u+k=x
x=u+k
u=x-k
add k to both sides
u+k=x-k+k
u+k=x+0
u+k=x
x=u+k