Subtract both sides of the expression, right hand side(10+k) and left hand side(4K), by -k as you want to get rid of the variable to get simply 10 on the LHS. 10+k -k = 4k -k
If we want to subtract k from k then we get 0, and if we want to subtract k from 4k we get 3k. For the sake of simplicity think of k as an object saying if I subtract object A from object A I get zero, and so forth.
We are left with the expression: 10 = 3k We divide both sides by 3 as to get rid of it from the RHS. We get a solution 10/3 = k
