Explain why these two equations have the same solution. Do not calculate the solution.

Question

3 × m + 4 = 25 and 3 × m = 21

Answer 1

3m + 4 = 25 <--- the original equation
3m = 25-4
3m = 21 <----- the equation after subtracting 4 from both sides

This is a proper step to take, so the two equations are equivalent.