3 · {(300 - 80 ÷ 5) - [3 · 25 - (8 - 2 · 3)]} I have no idea what this is or how to solve it.

3 · {(300 - 80 ÷ 5) - [3 · 25 - (8 - 2 · 3)]} 

 

$$\\=3 \times \{(300 - 80 \div 5) - [3 \times 25 - (8 - 2 \times 3)]\}\\\\
$Do the stuff in the brackets, and use PEDMAS $\\
$Multiplication and division is done before addition and subtraction$\\\\
=3 \times \{(300 - 16) - [3 \times 25 - (8 - 6)]\}\\\\
=3 \times \{(284) - [3 \times 25 - (2)]\}\\\\
=3 \times \{284 - [3 \times 25 - 2]\}\\\\
=3 \times \{284 - [75 - 2]\}\\\\
=3 \times \{284 - [73]\}\\\\
=3 \times \{284 - 73\}\\\\
=3 \times \{211\}\\\\
=3 \times 211\\\\
=633$$