x+y=-4
(x-2)^2+(y+7)^2=13
solve by substitution
solve the first equation for either variable. For example, solve it for x.
x + y = -4
x = -y - 4
Then, where ever that variable appears in the second equation, replace it with the value from the first equation that is equal to it. This will allow you to find the value of the other variable. Following the original example, where the first equation was solved for x.
(x - 2)^2 + (y + 7)^2 = 13
([-y - 4]^2 + (y + 7)^2 = 13
Can you take it from there?