The second term of an arithmetic sequence is 24 and 5th term is 3. find the first term and the common difference.

We know that
the formula for the arithmetic sequence is 
an=a1+d(n-1)

for a2=24  n=2
24=a1+d(2-1)> 24=a1+d> equation 1

for a5=3  n=5
3=a1+d(5-1)>3=a1+4d> equation 2

multiply equation 1 by -1  and adds to  equation 2
-24=-a1-d
3=a1+4d

-21=3d
d=-7
24=a1+d> 24=a1-7> a1=24+7> a1=31

the answer is
the first term is 31 and the common difference is -7

What am i doing here. add?