Round (a) 0.002 864 3 to three significant figures, (b) 456 780 to three significant figures, in…

Step 1. (a) The figures are $2$, $8$, $6$, and the next digit is $4$: round down to $0.00286$.

Step 2. (b) $456\,780$: keep $4$, $5$, $6$; the next digit is $7$, so round up: $457\,000 = 4.57 \times 10^{5}$.

Step 3. (c) $6.951$ to two figures: $6.9$ and the next digit $5$ (then $1$) rounds up, giving $7.0$.