(1/4×(-7,2)+1/8×(-7,2))×(-7,2^2)/2

(-7,2)(1/4+1/8)*(-7,2²)*1/2=(-7,2)³(1/4+1/8)*1/2=(-7,2)³*3/8*1/2=(-7,2)³3/16=-(36/5)³*3/16=-46656*3/(125*16)=-139968/2000=-69,984