Please help photomaths isn't working and I don't get how to do this

Our functions are:
[tex]f(x) = -5x + 12\\g(x) = -\frac{3}{2}x + 4\\[/tex]
To solve this, we must use the expression g(-10) - f(3).
[tex]-\frac{3}{2}(-10) + 12 - [-5(3)] + 12\\\\15 + 12 - [-5(3)] + 12\\\\15 + 12 - (-15) + 12\\\\27 - (-15) + 12\\\\42 + 12\\\\54[/tex]
The value of the expression g(-10) - f(3) is 15.