.
Although the post is silent on what Dy is, I will assume that Dy is the determinant of a matrix
from the "Cramer's rule" / "Cramer's solution".
Solution
Step 1. Write the system in the canonical form
-4x + 7y = 1
2x + 5y = 25
Step 2. Write the coefficient matrix of the system
A =
Step 3. In matrix A, replace the second column by the right side vector
Ay = .
Step 4. Now Dy is the determinant of the matrix Ay
Dy = det(Ay) = -4*25 - 2*1 = - 100 - 2 = -102. ANSWER
Solved.
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On Cramer's rule for solving systems of 2 equations in 2 unknowns see the lessons
- What is a matrix?
- Determinant of a 2x2-matrix
- HOW TO solve system of linear equations in two unknowns using determinant (Cramer's rule) (*)
- Solving systems of linear equations in two unknowns using the Cramer's rule (**)
in this site.
The most relevant / closest lessons are marked in the list as (*) and (**).
Also, you have this free of charge online textbook in ALGEBRA-II in this site
- ALGEBRA-II - YOUR ONLINE TEXTBOOK.
The referred lessons are the part of this online textbook under the topic
"2x2-Matrices, determinants, Cramer's rule for systems in two unknowns"