How is this solved. ive tried different ways and cant get it

It's kinda like using variables, but shapes in their place. Let's set the circle to x, the triangle to y, and the square to z. Now the equations say x+(y+z)=13, (y+z)=8, and x+y=7. First thing we can do it plug 8 into the first equation for y+z, getting x+8=13. Subtract 8 to both sides and x (or circle) equals 5. Plug 5 into the last equation, x+y=7 to solve for y, getting 5+y=7. Subtract 5 to both sides and y (or triangle) equals 2. Finally, plug two in for y in y+z=8, getting 2+z=8. Subtract 2 from both sides and z (or square) equals 6. Hope I've explained it clearly!!

The correct answer will be d. -4