Ignore the negative sign, since that only affects the sign of the final product
3415₇ × 432₇
You can compute the product by considering the digit expansion, by which I mean first expanding each number as sums of powers of 7:
3415₇ = 3000₇ + 400₇ + 10₇ + 5₇
(It's the same thing as writing, for example, the decimal 3415 as 3000 + 400 + 10 + 5, just in a different base.)
Similarly,
432₇ = 400₇ + 30₇ + 2₇
Then apply the distributive property:
= (3000₇ + 400₇ + 10₇ + 5₇) × (400₇ + 30₇ + 2₇)
= 3000₇ × (400₇ + 30₇ + 2₇)
… + 400₇ × (400₇ + 30₇ + 2₇)
… + 10₇ × (400₇ + 30₇ + 2₇)
… + 5₇ × (400₇ + 30₇ + 2₇)
= {12}00000₇ + {9}0000₇ + 6000₇
… + {16}0000₇ + {12}000₇ + {8}00₇
… + 4000₇ + 300₇ + 20₇
… + {20}00₇ + {15}0₇ + {10}₇
where I use the curly braces to indicate products (written in base 10) of digits that exceed 6. Anything in these braces needs to be properly converted to base 7. For example,
{12}₇ = 7 + 5 = 15₇
So you would have
= 1500000₇ + 120000₇ + 6000₇
… + 220000₇ + 15000₇ + 1100₇
… + 4000₇ + 300₇ + 20₇
… + 2600₇ + 210₇ + 13₇
Add everything up:
• In the 7⁰ place,
3₇
• In the 7¹ place,
1₇ + 1₇ + 2₇ = 4₇
• In the 7² place,
2₇ + 6₇ + 3₇ + 1₇ = {12}₇ = <u>1</u>5₇
• Carry the 1 (underlined). In the 7³ place,
2₇ + 4₇ + 1₇ + 5₇ + 6₇ + <u>1₇</u> = {19}₇ = <u>2</u>5₇
• Carry the 2. In the 7⁴ place,
1₇ + 2₇ + 2₇ + <u>2₇</u> = {7}₇ = <u>1</u>0₇
• Carry the 1. In the 7⁵ place,
2₇ + 1₇ + 5₇ + <u>1₇</u> = {9}₇ = <u>1</u>2₇
• Carry the 1. In the 7⁶ place,
1₇ + <u>1₇</u> = 2₇
So, we end up with
3415₇ × 432₇ = 22045543₇
and thus
3415₇ × (-432₇) = -22045543₇
