2.1(a+0.5)=-2.1(a+20)

Solution for 2.1(a+0.5)=-2.1(a+20) equation:

2.1(a + 0.5)=-2.1(a+20)

We simplify the equation to the form, which is simple to understand 
2.1(a + 0.5)=-2.1(a+20)

Reorder the terms in parentheses
+(+2.1a+1.05)=-2.1*(a+20)

Remove unnecessary parentheses
+2.1a+1.05=-2.1+*(+a+20+)

Reorder the terms in parentheses
+2.1a+1.05=+(-2.1a-42)

Remove unnecessary parentheses
+2.1a+1.05=-2.1a-42

We move all terms containing a to the left and all other terms to the right. 
+2.1a+2.1a=-42-1.05

We simplify left and right side of the equation. 
+4.2a=-43.05

We divide both sides of the equation by 4.2 to get a. 
a=-10.25