Sum of 3

Let $a =$ pink circle, $b =$ green square, and $c =$ yellow star.

$2a + b = 7$

$2b + c = 19$

$2c + a = 19$

Adding all of the equations yields:

$3a + 3b + 3c = 45$

$3(a + b + c) = 45$

$a + b + c = 15$

suppose , circle = x ; square =y and star =z

so , 2x +y =7..........................(1)

      2y +z =19 ......................(2)

     2z +x =19.....................(3)

so, (1) + ( 2) + (3) =2x +y + 2y +z +2z +x =45

                      or, 3( x+y +z)    =45

                          or. x+ y +z  =15

We can solve it using three equations with three unknowns.

Let each green square be $x$

Let each red circle be $y$

and each star be $z$ ,

then we have

$2y + x = 7$ ,

$2x + z= 19$ ,

$2z+ y= 19$ . Solving this system yields $y=1, x=5 \text{ and } \ z=9 \implies x+y+z=15$