I'm a mathematician at heart, I don't like shapes, they confuse me like they do to other people... so I convert it all into numbers, so when ever I get one, I see it more of a maths question than a logic question. It makes it easier to go about.
With the first equation, imagine the triangle is equal to one, therefore you're going to convert every shape into terms of the triangle. As 4 triangles equal the rectangle, 1+1+1+1 = 4. Therefore, rectangle = 4.
Let's focus on trying to work out the square. Do this by using simultaneous equations, and putting the second equation, equally circle, into the third equation. Therefore you have 3 squares = 3 triangles + square.
As you know triangle = 1, and using basic maths (minusing a square from each side), you now end up with 2 squares = 3, and therefore square = 3/2.
Put the value of square= 3/2 and triangle=1 back into the equation, and you get that circle = 7/2.
Put all these values into the last equation:
7/2 + 7/2 = 4 + 4 -?
7 = 8 -?
Therefore ?=1. Therefore as triangle =1, the answer is triangle.
In writing it seems like it takes a while, but for me, I can do it relatively quickly, and this one seems to be a slightly harder one compared to usual.
I don't know if this is any help, but I thought I'd share how I do these questions