1/2 - n = 3/8
Subtract 1/2 from both sides of the equation:
-n = 3/8 - (1/2)
Then you need to make the denominator (the number on the bottom of the fractions) the same:
-n = 3/8 - (1/2)x(4/4) = 3/8 - (4/8) = (3 - 4)/8 = -1/8
Now you can divide both sides by negative 1 to get:
n = 1/8