Answer:
15
General Formulas and Concepts:
Pre-Algebra
Order of Operations: BPEMDAS 
- Brackets 
- Parenthesis 
- Exponents 
- Multiplication 
- Division 
- Addition 
- Subtraction 
Equality Properties 
- Multiplication Property of Equality 
- Division Property of Equality 
- Addition Property of Equality 
- Subtraction Property of Equality 
Explanation:
Step 1: Define
Identify
5a - 10b = 45
b = 3
Step 2: Solve for a
- Substitute in b [Equation]: 5a - 10(3) = 45
- Multiply: 5a - 30 = 45
- [Addition Property of Equality] Add 30 on both sides: 5a = 75
- [Division Property of Equality] Divide 5 on both sides: a = 15