Solving a simple Partial Differential Equation

[Topper]

Hi,
I am trying to find the right approach to 1.) partially differentiate an equation and then 2.) solve the partial differential equation in next step with to be defined numbers for the variables:

Simple example equation:
(1/2)*(a^3+4*b^4-c*a^2)

1.) partially differentiate for variable a (result should be: (1/2*(3a^2-2ac))
2.) solve the resulting partial differential equation for a=3, b=5 and c=4 (result: 1.5)

The sample above is easy to solve, but if the partial differential equation is very complex, it is very time consuming to retype everything with the defined numbers for the variables.
So what would be the fastet way to solve both steps within one command in the HP Prime?
Thanks for your help!

[phobos]

subst(diff((1/2)*(a^3+4*b^4-c*a^2),a),[a = 3,b = 5,c = 4])

[Topper]

Thank you phobos! This was what I was looking for, this really helps a lot!
One small correction with the braces for the variables:
subst(diff((1/2)*(a^3+4*b^4-c*a^2),a),{a = 3,b = 5,c = 4})