2c-d=-2
4c+d=20
taking the equation 2c-d=-2 make d the subject of the formula which is
d=2c+2
substitute d with the above equatio in the second equation 4c+d=20 then you will get
4c+2c+2=20 then solve thie equation and get
6c+2=20 subtract 2 both sides in this equation and get 6c=18 and divide by 6 both sides and get
c=3
take the first equation 2c-d=-2 and substitute c with 3 like this 2*3-d=-2 and get 6-d=-2 and find d
subtract 6 both sides and remain with -d=-8 and divide by -1 both sides and get d=8