You would use the formula for the specific term you wish to find;
The formula is:

a = starting value of the sequence
d = the common difference (i.e. the difference between any two consecutive terms of the sequence)
n = the value corresponding to the position of the desired term in the sequence (i.e. 1 is the first term, 2 is the second, etc.)
Un = the actual vaue of the the term
For example, if we have the arithmetic sequence:
2, 6, 10, 14, ...
And let's say we want to find the 62nd term;
Then:
a = 2
d = 4
(i.e. 6 - 2 = 4, 10 - 6 = 4, 14 - 10 = 4;
You should always get the same number no matter which two terms you find the difference between so long as they are both
consecutive [next to each other], otherwise you are not dealing with an arithmetic sequence)
n = 62
And so:
Okay, so since both y values share the same number, let's add both equations so the y values cancel out leaving us to solve for x.
10x+(-5x)=5x
7y+(-7y)=0
1+24=25
Now we have 5x=25
divide both sides by 5 and we have x=5
Now that we know x=5, we can plug it into one of the problems to find y.
Let's do 10(5) +7y=1
50+7y=1 subtract 5 and we get 7y=-49
divide -7 from both sides and we get y=-7
so there you have it x=5 and y=-7
C. $34.50. A car gets 30 miles per gallon, if the gasoline cost $1.15 per gallon, a 900 miles trip will cost $34.50
This exercise can be solved by simply arithmetic.
Divide the total miles by the miles per gallon:
900/30 = 30
Then, multiply this value by the cost per gallon:
30 x $1.15 = $34.50