−21=−3−2a 2

Sure, let's solve the equation step by step:

The given equation is: −21 = −3 - 2a

Step 1: Add 3 to both sides of the equation to isolate the term with the variable 'a' on one side. This gives us:

-21 + 3 = -3 + 3 - 2a

Which simplifies to:

-18 = -2a

Step 2: Now, divide both sides of the equation by -2 to solve for 'a'. This gives us:

-18 / -2 = -2a / -2

Which simplifies to:

a = 9

So, the solution to the equation −21 = −3 - 2a is a = 9.