We are given equation : mx =5.
We need to solve the equation for m first.
m is being multiplied with x.
So, we need to get rid m from left side.
The reverse operation of multiplication is division, so we need to divide both sides by m, we get

x =
.
Please see, we have m in denominator.
So, we can't have 0 in denominator.
For all other each real numbers x would have a single solution.
Therefore, solution for m is : All real numbers except 0.
And for m=0 the equation does not have solutions.
And for m=x/5, we have infinite number of solutions.