I am a complete beginner in MathCAD and would really appreciate if somebody would help me define this with the values I provided. I tried to define the function but it just wouldn't work and I'm not sure what I'm doing wrong.
It looks like you are defining your variables differently than you have them in the equations. Your definitions do not have subscripts.
for h1a, you should have typed h<ctrl ->1a to get the subscripted variable as you have it in the equation.
For future reference, it is best to attach a Mathcad document with the example problem. Other users can more easily debug and recommend changes.
I tried to define the function but it just wouldn't work and I'm not sure what I'm doing wrong.
Neither can we say what you did wrong as we don't see what you tried.
Attach the worksheet with the defined values and also with the function you tried to define. We don't see in the picture how the function f_H.R is defined anyway. So no one could give you the desired value of m.H.R.