If b = 3 you substitute al the b's on the equation by 3:
5a - 10(3) = 45 - you substitute b for 3
5a - 30 = 45 - you do 10 * 3
5a = 45 + 30 - you move 30 to the right side of the equation
5a = 75 - you do 45 + 30
a = [tex] \frac{75}{5} [/tex] - you take 5 to the right side of the equation
a = 15
Hope it helped,
Happy homework/ study/ exam!
