To solve these equations, we first need to isolate the variable in each equation.

For the first equation:

1) mx = 20
To isolate the variable m, we divide both sides by x:
m = 20/x

For the second equation:

2) (m+3)x = -18
To isolate the variable m, we first need to distribute the x:
mx + 3x = -18
Next, we can substitute the value of m from the first equation into the second equation:
(20/x)(x) + 3x = -18
20 + 3x = -18
Now, we can isolate x:
3x = -18 - 20
3x = -38
x = -38/3
x = -12.67

So, the solution for the system of equations is m = 20/x and x = -12.67.