/sci/ - Science & Math

Can someone show me how to use a determinant of one matrix to find the determinant of another?

I can calculate determinants and the cramer rule and stuff but i'm not sure how to set up the question.

What i have:

[-1 2 1] [matrix1] = [matrix 2]

aei + bfg + cdh - gei - hfa - idb = 2

but i don't know what i'm supposed to do to get the answer of -4.