a_n = a1+ d* (n-1)$$a_{n}$$ = $$a1+d\cdot (n-1)$$

a1+ d* (n-1) = a_n$$a1+d\cdot (n-1)$$ = $$a_{n}$$

a1 = a_n- d* (n-1)$$a1$$ = $$a_{n}-d\cdot (n-1)$$

a1 = a_n- n* d+d$$a1$$ = $$a_{n}-n\cdot d+d$$

a1 = a_n- n* 1.5+1.5$$a1$$ = $$a_{n}-n\cdot 1.5+1.5$$

a1 = a_n- 1.5* n+1.5$$a1$$ = $$a_{n}-1.5\cdot n+1.5$$
