Question 1008696
Call the numbers x and y.  Thus we have
x = y + 3   and
x^2 - y^2 = 51
Now substitute and get
(y+3)^2 - y^2 = 51
y^2 + 6y + 9 - y^2 = 51
6y + 9 = 51
6y = 42
y = 7
x = 10