you have to set x to itself but there are 2 x so you have to set one to the other side, normally you will switch the smallest one in this case it is -13x so since its a negative lets add it t both sides(the only way to get rid of a negative is with a positive and what you do to one side you have to do it to the other) now you have -1x+7=13 you have to subtract the 7 to leave x by itself this gives you -1x=6, now finally divide by -1 in both sides(because the only way to get rid of a multiplication is with a dividion) this will result in x=-6
Hope this helps