We must follow order of operations rules here. The exponentiation must be done first, followed by the multiplication; only then does subtraction happen.
Subbing 1 for y in 5xy^2 - 3, we get 5x - 3. Subbing 5 for x, we get 5(5)-3=22.
The value of the given expesion at x=5 and y=1 is 22.