If x=5 , then x+7 = 5+7 .

Question

How i can do this ?

Answer 1

It is already done.  Here is how it is done (and usually presented):

 

Basically you have a relationship  x+7=y.

The problem asks " If x = 5, what is y?"

 

Do the problem by substituting the value 5 for x in the original equation, ie:

x+7=y

5+7=12=y.

This fundamental algebra.