Solve the equation for the red variable

I think c = p+ r because you firstly have to rearrange the equation so that you isolate the -c. which means you get -c = p-r because you have to change the sign of the r when you switch it. so -c = p-r, but to find c that isn’t minus you need to change the sign on both sides. therefore, the answer should be c = p+r.