Not easily done in my head but I'm gonna try (no calculator!)
56 * 7 = 7*6 + 50*7 = 42 + 350 = 392
392 / 4 = (400/4) - (8/4) = 98
Now the hard part, 98^6.
98 * 98 = 8 * 8 + 2*90*8 + 90 * 90 = 64 + 1440 + 8100 = 9604
9604 * 9604 (98^4) = (9000 + 600 + 4) * (9000 + 600 + 4) = 9000*9000 + 9000*600 + 9000*4 + 600*9000 + 600*600 + 600*4 + 4*9000 + 4*600 + 4*4 = 9000*9000 + 600*600 + 4*4 + 2*(9000*4 + 600*9000 + 600*4)
= 81,000,000 + 360,000 + 16 + 2*(36,000 + 5,400,000 + 2,400) = 81,360,016 + 10,876,800 = 92,236,816
Last one
92,236,816 * 9604 = 6*9,604 + 10*9,604 + 100*8*9604 + 1,000*6*9,604 + 10,000*3*9,604 + 100,000*2*9,604 + 1,000,000*2*9,604 + 10,000,000*9*9604
= 57,624 + 96,040 + 7,683,200 + 57,624,000 + 288,120,000 + 1,920,800,000 + 19,208,000,000 + 864,360,000,000
= 153,664 + 65,307,200 + 2,208,920,000 + 883,568,000,000
= 65,460,864 + 885,776,920,000
= 885,842,380,864
Finally we have
885,842,380,864 - 20 = 885,842,380,844
Thus the answer is 885,842,380,844
Checked on Google and this is correct, phew, so many numbers, so much double checking.
Do you like math?