Explanation

given $U_4$ = 15 = a + 3d and 3 + r + s + ..

here a = 3

15 = 3 + 3d

12 = 3d, d = 4

thus

$3^{rd}$ term $U_3 = a + 2d$

$U_3 = 3 + 2(4)$

= 3 + 8 = 11

given $U_4$ = 15 = a + 3d and 3 + r + s + ..

here a = 3

15 = 3 + 3d

12 = 3d, d = 4

thus

$3^{rd}$ term $U_3 = a + 2d$

$U_3 = 3 + 2(4)$

= 3 + 8 = 11