You want to use PEMDAS for this.
P= arenthesis ( )
E= xponents a²
M= ultiply ×
D= ivide ÷
A= dd +
S= ubtract -
What you want to do is go in order and ask yourself as you go.
Are there any Parenthesis? No
Are there any Exponents? No
Is there any Multiplication? Yes
Is there any Division? No
Is there any Addition? No
Is there any Subtraction? Yes
Thats when you stop and you multiply what you have in the PEMDAS. Top to bottom. 3x5 which gives you 15. Now you equation is 45-15. Thats when you subtract and you get your answer which is 30.