Remember the Rules of Operations (PEDMAS):
Parentheses
Exponents
Multiplication
Division
Addition
Subtraction
So with the expression 3 + 4 x 3 - 3 x 5 we know to multiply first. So let's multiply 4 x 3 and -3 x 5 first:
3 + 4 x 3 - 3 x 5
= 3 + 12 - 15
Then let's work out the rest of the expression!
3 + 12 - 15
= 15 - 15
= 0
Therefore we can determine that 3 + 4 x 3 - 3 x 5 = 0
I hope this helped! :)