How to do this maths question?

Question

the sum of the 4th and the 5th term of an arithmetic sequence is 18.if the sum of the first two terms is 42,find the general term and the 15th term.

Answer 1

n th term is given by the formula

Tn = a + (n – 1)d

T4 = a + (4 – 1)d = a + 3d

similarly

T5 = a + 4d

T4 + T5 = a + 3d + a + 4d  = 2a + 7d = 18 ---------- eq.1

T2 = a + d

T1 + T2 = a + a + d = 2a + d = 42 ---------- eq.2

eq.1 – eq.2

6d = – 24

d = – 4

from eq. 2

2a – 4 = 42

a = 23

General term Tn = 23 –  4(n – 1)

T15 = 23 – 4 × 14 = – 33

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Answer 2

2 variables, first term a, difference d

a + (a+d) = 42

(a + 3d) + (a + 4d) = 18

2a + d = 42

2a + 7d = 18

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-6d = 24

d = -4

so 2a - 4 = 42, 2a = 46, a = 23

general term a_n = 23 - 4(n-1) = 27 - 4n

15th term a_15 = 23 - 4(14) = 23 - 56 = -33