v01=10
v02=15
s=750
a1=6
a2=4

rov1=Solve[s==0.5 a1 t^2+v01 t+0.5 a2 t^2+v02 t,t]

t1=t/.rov1[[2]]

Plot[{a1 t+v01,a2 t+v02},{t, 0,t1}, AxesOrigin->{0,0}, AxesLabel->{"t(sec)","v(m/s)"},PlotStyle->{Green,Red}]

ClearAll[s]

v1=60/3.6
v2=40/3.6
s=100

vys=Solve[s==v1 ((v1-v2)/a)-0.5 a ((v1-v2)/a)^2,a]

{{a->0.7716049382716053`}}
Vysledek=a/.vys[[1]]

({{\[Placeholder]}, {\[Placeholder]}})

h=78.5
g=10

rov=Solve[h==0.5 g t^2,t]

t1=t/.rov[[2]]

vys=Solve[x==0.5 g (t1-1)^2,x]

{{x->43.87677448768213`}}

x1=x/.vys

vysledek=h-x1

Plot[h-0.5 g t^2,{t,0,t1}, AxesOrigin->{0,0}]