-3*4.2=a

Solution for -3*4.2=a equation:

Simplifying
-3 * 4.2 = a

Multiply -3 * 4.2
-12.6 = a

Solving
-12.6 = a

Solving for variable 'a'.

Move all terms containing a to the left, all other terms to the right.

Add '-1a' to each side of the equation.
-12.6 + -1a = a + -1a

Combine like terms: a + -1a = 0
-12.6 + -1a = 0

Add '12.6' to each side of the equation.
-12.6 + 12.6 + -1a = 0 + 12.6

Combine like terms: -12.6 + 12.6 = 0.0
0.0 + -1a = 0 + 12.6
-1a = 0 + 12.6

Combine like terms: 0 + 12.6 = 12.6
-1a = 12.6

Divide each side by '-1'.
a = -12.6

Simplifying
a = -12.6