That "l" looks too much like the number "1", so I'm going to 
use capital letters instead:

S = N(A + L)/2

Clear of fractions by multiplying both sides by 2

2S = 2N(A + L)/2

Then the 2's cancel on the right

2S = 2N(A + L)/2

and then we don't have any fractions:

2S = N(A + L)

Now remove the parentheses using the distributive principle:

2S = NA + NL

Now isolate the only term that contains L on the right by
subtracting NA from both sides:

2S - NA = NL

Finally we divide both sides by N which is the coefficient of L:

 = L

And if you like you can turn the equation around and write it as

L =  

Edwin