calculate how many address is there in /29 subnet (full IP has 32 bits, so you're left with 3 bits for subnet), 2 to the power of 3 is 8. You have 8 address per subnet.
see how many /29 subnets can you have in a one /24 subnet (172.16.0.0-172.16.0.255), it 256/8 so 32 /29 subnets
Divide 300 by 32 and get 9.75 so the 300th /29 subnet will be in the 10th /24 subnet (172.16.9.0-172.16.9.255), beacuse you have to take 9 full /24 subnets and part of 10th
multiply 9*32 and you get that the 9th subnet ends with 288th /29 subnet, so you have to take 12 more /29 subnets from the 172.16.9.0/24, which is (12-1)*8 so the 300th is 172.16.9.88.0/29
You calculate it. A /29 subnet has 8 addresses (3 bit host address), so how many addresses do n-1 subnets have? And how many of those addresses fit in one octet?