Hey there!
To solve this expression, you must follow your orders of operations or PEMDAS (Parenthesis, Exponents, Multiplication and Division, Addition and Subtraction).
In the case of 5[3(14.5+7)-3]-7.4, you not only have parenthesis, but you also have brackets. To start, you would first simplify what is in the parenthesis:
5[3(14.5+7)-3]-7.4
5[3(21.5)-3]-7.4
Now, simplify what is in the bracket, starting by multiplying 3(21.5) since multiplication comes before addition and subtraction:
5[3(21.5)-3]-7.4
5[64.5-3]-7.4
5[61.5]-7.4
Now, multiply 5*61.5 since multiplication comes next:
5[61.5]-7.4
307.5-7.4
And finally, subtract what is left since it is your final operation remaining to get 300.1 as your final answer.
Hope this helps and have a marvelous day!