Since y=5 the equation becomes:
5=1/2x-3
Add 3 to both sides
5+3=8, -3+3=0
Now you have:
8=1/2x
Multiply 2 (since the opposite of 1/2 is 2/1) by both sides
8x2=16, 1/2x2/1=2/2 or 1
Now you have:
16=x
When you check it:
5=1/2(16)-3
5=(16/2)-3
5=8-3
Hope this is not too confusing and it helps!