Answer:
a = r + d - c
Explanation:
a + c = r + d
You want a alone on the left side. c is being added to a.
To get rid of the c, you must subtract c from the left side, but the rule of equations is that you must do the same operation to both sides of the equation. We subtract c from both sides of the equation.
a + c - c = r + d - c
c - c = 0, so the c is eliminated from the left side.
We now have
a + 0 = r + d - c
a = r + d - c