What is the value of a in the equation 5a – 10b = 45, when b = 3?

To get the answer we have to substitute b=3 and solve it.
5a-10b=45
5a-10*3=45
5a-30=45         /+30  add 30 both sides
5a-30+30=45+30
5a=75              /:5    divide both sides by 5
5a:5=75:5
a=15 - its the answer

Sis less than or equal to 75 (bottom left)