number divisible by 3 and 5 so number devisible 15 .                                                             But number smaller than 50 are greater than 40 devisible 15 only can 45

f(7) = p7⁷  + q7³ + r7 - 4       ;         f(-7)= p (-7)⁷  + q (-7)³  +  r(-7) - 4

f(7) + f(-7) = -8  =>  f(7)  = -11