I'll be referring to each of these numbers as x and y.
x + y = 4
(x^2) + (y^2) - 3(x)(y) = 76
x = 4 - y
(4 - y)^2 + (y^2) - 3(4 - y)(y) = 76
(4 - y)(4 - y) + y^2 - (3y)(4 - y) = 76
16 - 4y - 4y + y^2 + y^2 - 12y + 3y^2 = 76
16 - 20y + 5y^2 = 76
5y^2 - 20y - 60 = 0
y^2 - 4y - 12y = 0
(y - 6)(y + 2) = 0
y = 6 or -2
x = 4 - 6 = -2
x = 4 - - 2 = 6
As you can see, we got the same numbers for both x and y, -2 and 6. Therefore, the two numbers are -2 and 6. But, we can check our work to ensure that the answer is correct.
x = -2
y = 6
6 - 2 = 4
4 = 4
(-2)^2 + (6^2) - 3(-2)(6) = 76
4 + 36 - 3(-12) = 76
40 + 36 = 76
76 = 76
Hope this helps!