Hi seal,
welcome to the FCC forum!
Since you are on the last exercise, I assume it is obvious to you that we are expected to use the shorthand grid-template: here. In the previous exercises it was defined as:
grid-template: rows / columns;
In pseudocode we could rephrase the instructions as:
row 1: greatest available height
row 2: 50px
column 1: one fifth of the total width
column 2: four fifths of the total width
Remember that in exercise 23 we got introduced to the new ‘fraction’ unit. It is useful to express values as described in row 1 and the columns.