You can put this solution on YOUR website! "[" this is the bracket
it works like a parentheses
the parentheses indicates priority in a equation
the numbers that are inside it need be solved first
the bracket comes after of the parentheses
so first you solve the parentheses and later the bracket
4[3(21-17)+3]
4[3(4)+3]
4[3*4+3]
4[12+3]
4[15]
4*15
60
and after of the braket still exist the "{" it is the third and last to be solved
