Square+circle =9.2 (2)

Triangle+circle = triangle+triangle+triangle (3)

( 1)+(2)

Triangle+circle = 12 (4)

Replace (4) with (3)

Three triangles = 12

A triangle =4 (5)

Substitute (5) for (1)

Square =4-2.8= 1.2

Replace (4) with (5)

Circle = 12-4=8

So: triangle =(4), square =( 1.2), circle =(8).