SOLUTION: 1. The 100th Term is -354 the difference is -6 whats the first term? I dont understand how to even start this. I don't even understand the An=a1+d(n-10 and I dont understand Sn=

It's all plugging numbers for letters in formulas.
You just have to know what they mean.

So n=100 and an = a100 = -354  

So d=-6

So a1 is what we're looking for

So we look through our storehouse of formulas 
for arithmetic sequences and series.

an = a1 + (n-1)d

Let's substitute 100 for n:

a100 = a1 + (100-1)d

Let's substitute -354 for a100:

-354 = a1 + (100-1)d

Let's substitute (-6) for d

-354 = a1 + (100-1)(-6)

Subtract 100-1

-354 = a1 + (99)(-6)

Multiply the 99 by the -6 and get -594

-354 = a1 - 594

Add 594 to both sides,

-354 = a1 - 594
+594       +594  
----------------
 240 = a1

So the first term, a1, is 240.

---------------

To check it we write out all 100 terms, starting
with 240 and adding -6 each time (same as 
subtracting 6) and see if when we get to the 100th number,
it will be -354.  We'll make 10 rows and 10 columns going
across:

 240  234  228  222  216  210  204  198  192  186
 180  174  168  162  156  150  144  138  132  126
 120  114  108  102   96   90   84   78   72   66
  60   54   48   42   36   30   24   18   12    6
   0   -6  -12  -18  -24  -30  -36  -42  -48  -54
 -60  -66  -72  -78  -84  -90  -96 -102 -108 -114
-120 -126 -132 -138 -144 -150 -156 -162 -168 -174
-180 -186 -192 -198 -204 -210 -216 -222 -228 -234
-240 -246 -252 -258 -264 -270 -276 -282 -288 -294
-300 -306 -312 -318 -324 -330 -336 -342 -348 -354
      
So we ended up with -354 as the 100th term in the 
lower right hand corner, so we know 240 was the 
correct first term a1.  But it does take a 
very long time to check them, so we just have to be
careful with our calculations, and not check them.

-------------------------------------------------

The other formula you asked about:

Sn = (a1 + an)

 That is a formula for Sn, where S100 is the 
sum of all those 100 numbers if you were to add them all together:

There is another formula for the sum. It is:

Sn = [2a1 + (n-1)d]

Edwin