I would like to be able to solve the following for the unknowns a, b, c and d.

a*4 + b*2 + c *9 + d*7 = 21
a*2 + b*5 + c*4 + d*4 = 11.5
a*6 + b*3 + c*5 + d*3 = 15.5
a*8 + b*4