First step is to get number of student that take all three class. which is
100-(42+34+58-8-13-18) =5

Now out of 8 students who are science and law graduates and 18 students who are are commerce and science graduates there is 5 of them who took all three so they are counted twice.

so 42 - (18 + 8 - 5) =21

100 = S+ C + L - ( SC + SL + LC ) + SLC
100 = 42+34+58 - ( 39 ) + SLC

SLC = 5

Since we want to find only Science Graduates:

= 42 + SLC - (SL+SC)
= 42 + 5 - 26
= 21

ALTERNATIVELY:

If you look at the second equation, you will realize that we can insert it into the first one

42 + SLC - (SL+SC) = is what questions asks us (LET'S CALL IT X) and
100 = S+ C + L - ( SC + SL + LC ) + SLC is the first equation

So the question actually boils down to:

100 = X + C + L - LC

100 = X + 58 + 34 - 13

X = 21

And this is how I remember the rules, hope this helps!