b-3/15=-1
We simplify the equation to the form, which is simple to understand 
b-3/15=-1
Simplifying:
b-0.2=-1
We move all terms containing b to the left and all other terms to the right. 
+1b=-1 + 0.2
We simplify left and right side of the equation. 
+1b=-0.8
We divide both sides of the equation by 1 to get b. 
b=-0.8