exam Maths Puzzle Numerical Ability

I'm not sure how to solve this problem.

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exam Other Question

Find the rank and nullity of the matrix
1 2 1 -1
2 3 2c+2 1
2 4 c+1 -2
1 2 c csquared-2

for all possible values of the parameter c. i need help solving this equation

|-15+9| (absolute value)
im guessing the correct answer would be 6 but wanted to make sure