The shapes are being used instead of variables.
For problem 4; start with clue 2, then clue 3, then clue 4, and lastly do clue 4.
Clue 2
3 · square - 8 = 19
3 · square = 27 added 8 to both sides
square = 9 divided both sides by 3
Clue 3
2 · triangle + 3 = -3
2 · triangle = -6 subtracted 3 from both sides
triangle = -3 divided both sides by 2
Clue 4
square + triangle = 6
(9) + (-3) = 6 substituted solutions from Clue 2 and Clue 3
6 = 6 True statement confirms that Clues 2 & 3 are correct
Clue 1
star + square = 12
star + (9) = 12 substituted solution from Clue 2
star = 3 subtracted 9 from both sides
Answer: triangle = -3, star = 3, square = 9
For problem 5; do the same thing as problem 4 and start with Clue 2
Answers: triangle = -2, star = 10, square = 8