I suggest computing the time tend at which v=-20 m/s by hand, since it is so simple and then use
v0 = 46;
t0 = 2;
a = -9.81;
t = linspace(t0, tend, 50);
for ii = 1:length(t)
v(ii) = v0 + a*(t(ii) - t0);
end
Alternatively, you can use your while loop instead
v(1) = v0;
t(1) = t0;
dt = 0.1; % or any other value
ii = 1;
while v(ii) >= -20
ii = ii + 1;
t(ii) = t(ii - 1) + dt;
v(ii) = v(ii - 1) + a*(t(ii) - t0);
end
