Dear Shivam, as far as I remember this question it asks to find the relation between t1, t2 and u (not h). If you want to find relation between t1, t2 and h then you can relate equation 1 and equation 4. I am providing you with the complete answer.

When a body, that is thrown vertically with velocity u,  reaches a vertical distance h from ground at time t1,

Thus, we will have, h = u×t1 - (1/2)g×t1^2--- (1)

If H is maximum the height it reaches with time T, then we will have

H = u2 / (2g) --- (2)

T = u/g  ............................... (3)

During descending, if the object crosses the same spot which is at a height h above the ground,  then the vertical distance travelled during ascending will be ( H-h ). Thus -

(H - h) = (1/2) gt^2 ---(4)

where t is the time taken to travel from highest point to the distance (H-h) .

If t2 is the time for the object, when it starts from the ground, and reaches maximum height and  again crosses the spot which is at a distance h from ground,  then we have,

t = (t2 - T )  --- (5)

Writing Eqn.(4) by substituting for H, h and t using respective eqns. (2), (1), (3) and (5)

[ u2 / (2g) ] - u×t1 + (1/2)g×t12 = (1/2)g [ t^2 - (u/g) ]^2 --- (6)

After simplifying equation (6), we will get,

t1 + t2 = 2u/g

All the best

Hope this helps