Arithmetic Sequences Help

Note that in general an arithmetic sequence is 
\[ (0)      a, a+d, a+2d, a+3d, a+4d, \dots\]
and the $n$th term is given by 
\[(1)            a_n=a+(n-1)d. \]
With these in mind, we try to answer the questions. 
16) The 11th term is 53, using formula (1) we have  
\[a_{11}=a+(11-1)d=a+10d=53.        (3)\]
The sum of 5th and 7th term is 56. This means
\[a_5+a_7=(a+4d)+(a+6d)=2a+10d=56.    (4)\]
Paying attention to (3) and (4), we see that we have two equations and two unknowns that we need to solve. Substract (3) from (4) to get 
\[(4)-(3) \Rightarrow (2a+10d)-(a+10d)=56-53 \Rightarrow a=3.\]
Substituting $a=3$ in (3) we get 
\[3+10d=53 \Rightarrow 10d=50 \Rightarrow d=\frac{50}{10}=5 \Rightarrow d=5.\]
Utilizing (0) we write the first three terms of the sequence
\[3, 3+5, 3+2 (5), \dots\]
So the first three terms are 
\[3, 8, 13.\]
17) The sum of the first two terms is 15: 
\[a+(a+d)=15 \Rightarrow 2a+d=15.                (5)\]
The sum of the next two terms (3rd and 4th) is 43: 
\[(a+2d)+(a+3d)=43 \Rightarrow 2a+5d=43.             (6)\]
Subtracting equation (5) from equation (6) we get 
\[(6)-(5) \Rightarrow (2a+5d)-(2a+d)=43-15   \Rightarrow 4d=28 \Rightarrow d=7.\]
Substituting $d=7$ in equation (5) we get 
\[2a+7=15 \Rightarrow 2a=8 \Rightarrow a=4.\]
So the first three terms of the sequence are
\[a, a+d, a+2d, \dots\]
that is
\[4,4+7,4+2(7), \dots\]
So the first three terms are 
\[4, 11, 18.\]